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BANCROFT 
LIBRARY 

♦ 

THE  LIBRARY 
OF 

THE  UNIVERSITY 
OF  CALIFORNIA 

T«  km   Rickard  Meoorlal 
Collection 
Gift  of  Mrs.  Rickard. 


Digitized  by  the  Internet  Archive 

in  2007  with  funding  from 

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http://www.archive.org/details/estimatisamplingOOrickrich 


THE 

SAMPUNd  AND  ESTIMATION 

OF 

ORE  IN  A  NINE 

BY 

T.  A.  RICKARD    ^ 

Editor  of  The  Engrineering  and  Mining  Journal,  Member  of  the  American 

Institute  of  Mining  Engineers,  Member  of  the  Institution  of  Mining 

and  Metallurgy,  Honorary  Fellow  of  the  Geological  Society 

of  Australasia,  State  Geoloerist,  Colorado,  1895-1901, 

Author  of "  The  Stamp-Milling  of  Gold  Ores" 

THIRD  IMPRESSION 

New  York  and  London 
THE  ENGINEERING  AND  MINING  JOURNAL 

1906. 

COPYRIGHT,  1903, 

BY 

THE  ENGINEERING  AND  MINING  JOURNAL. 


PREFACE. 


The  republication  of  the  papers  and  consequent  discus- 
sion upon  the  sampling  of  ore  in  mines  and  the  estima- 
tion of  the  tonnage  available  needs  no  apology;  it  is  a 
subject  of  the  utmost  practical  importance.  It  is  safe  to 
say  that  the  reputation  of  mining  engineers  has  suffered 
more  from  the  neglect  of  this  branch  of  practice  than 
from  any  other  cause.  On  the  other  hand,  those  engi- 
neers who  have  won  a  deserved  reputation  for  excellence 
of  judgment  are  the  men  who,  as  a  rule,  have  learned 
early  in  their  career  how  to  take  samples  which  are  a 
trustworthy  index  to  the  value  of  large  bodies  of  ore.  In- 
ferences from  the  results  thus  obtained  are,  of  course,  as 
important  as  the  sampling  itself,  but  he  who  is  careless  in 
the  performance  of  one  duty  is  hardly  likely  to  be  cir- 
cumspect in  the  other.  There  are  not  many  data  bearing 
upon  the  present  and  future  productiveness  of  a  mine 
which  an  engineer  can  secure  at  first-hand;  all  the  more 
reason  for  the  exercise  of  vigilance  in  collecting  those 
which  are  available.  Such  statements  indeed  are  truisms 
to  the  experienced,  but  they  are  overlooked  with  a  fatal 
frequency. 

Allied  to  this  subject  is  the  question  of  the  terms  to  be 
employed  in  describing  the  variously  conclusive  evidence 
regarding  ore  in  a  mine.  The  different  terms  suggested  in 
the  course  of  the  discussion,  which  is  herewith  repro- 


SAMPLING    AND    ESTIMATION    OF    ORE   IN    A    MINE 

duced,  can  be  contrasted  with  the  definition  brought  for- 
ward by  the  council  of  the  Institution  of  Alining  and  Met- 
allurgy, London,  in  a  recent  circular.  It  is  not  likely  that 
the  majority  of  mining  engineers  will  ever  agree  to  con- 
fine themselves  to  the  use  of  exactly  the  same  words,  nor 
would  we  advocate  such  a  step,  because  the  imposition  of 
cast-iron  terms  based  upon  a  suppositious  uniformity  of 
conditions  is  calculated  to  cripple  the  eflfort  to  prepare 
accurate  descriptions  of  unlike  occurrences  of  ore.  No  tech- 
nical word  or  set  phrase  will  cover  the  varying  degree  of 
evidence  obtainable  concerning  the  tonnage  of  ore  in  ex- 
istence in  a  mine,  or  in  the  various  portions  of  it.  Neverthe- 
less, while  it  is  unlikely  that  every  one  will  agree  to  use 
exactly  the  same  terms,  it  is  becoming  evident,  as  the  light 
of  intelligence  is  thrown  upon  the  phrase,  "ore  in  sight," 
that  it  not  only  fails  to  be  descriptive,  but  it  is  misleading, 
especially  to  the  average  investor  in  mines  who  is  apt  to 
take  it  in  its  literal  and  most  obvious  sense.  In  its  ulti- 
mate significance  only  ore  in  the  bins  is  "in  sight" — that 
is,  your  ore  is  in  sight  when  you  have  actually  taken  it  out 
of  the  mine.  The  discussion,  however,  brings  out  very 
clearly  the  need  for  precision  of  language  in  mine  reports, 
and  whether  an  engineer  uses  one  term  or  another  he 
should  always  be  at  pains  to  explain  just  what  he  does 
mean.  After  all,  the  primary  purpose  of  a  report  as  made 
by  a  mining  engineer  is  to  state  facts  as  lucidly  as  possible 
and  in  terms  which  cannot  be  misunderstood  by  the  person 
at  whose  request  and  expense  it  has  been  prepared.  It 
is  better  to  go  into  details  of  explanation  than  to  mislead, 
unconsciously,  by  the  use  of  terms  which  are  capable  of 
several  shades  of  meaning. 

The  discussion  of  these  practical  matters  will,  we  trust, 
serve  to  emphasize  the  fact  that  the  estimation  of  the  ore- 
reserves  and  the  exercise  of  judgment  concerning  the  fu- 
ture prospects  of  a  mine  require  most  of  the  qualities 
which  make  for  engineering  sense,  and  that  the  work  of 
sampling,  plus  the  inferences  from  it,  epitomizes  those 
characteristics  which,  taken  together,  constitute  the  dif- 
ference between  a  good  and  a  bad  mining  engineer.    If 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

we  can  assist  those  who  are  beginning  their  professional 
career,  we  shall  be  amply  recompensed. 

These  discussions  may  point  out  pitfalls  to  be  avoided 
by  the  young  engineer.  If  he  profits  by  the  experience  of 
others,  he  will  be  exhibiting  a  noble  thrift;  if  he  disre- 
gards it,  he  will  be  guilty  of  a  prodigal  squandering,  for 
there  is  no  improvidence  so  pitiful  as  the  waste  of  ex- 
perience. 

T.  A.  RiCKARD, 

Editor  of  The  Engineering  and  Mining  Journal. 
New  York,  July  lo,  1903. 


CONTENTS. 


PAGE. 

Preface  T.  A.  Rickard  i 

The  Sampling  and  Estimation  of  Ore  in  a  Mine 

T.  A.  Rickard  9 

Introduction  9 

Determination  of  Costs II 

Determination  of  the  Average  Value  of  Ore 14 

Work  of  Sampling 17 

Size  of  Sample 19 

Reduction  of  Samples 23 

Precautions  in  Sampling 26 

Wrong  Methods  of  Sampling 31 

Calculations  After  Sampling 34 

The  Question  of  High  Assays 39 

Possible  Discrepancies  Between  Sampling  and  Mining...  44 

Estimation  of   Ore-Reserves 51 

Inferences  from  Sampling 55 

Future  Prospects  of  a  Mine 60 

Collateral  Evidence 71 

Conclusion 74 

Ore  Developed — A   Definition Philip  Argall  76 

Cost  Per  Ton  as  a  Basis  of  Mine  Valuation 

R.  Gilman  Brown  82 

Mine  Valuation /.  Parke  Channing  88 

Ore  Developed George  E.  Collins  93 

Discussion C.   W.  Purington  96 

Discussion Philip  Argall  102 

Size   of   Sample 103 

Marking  Position  of  Samples 103 

Examining  the  Places  to  be  Sampled 104 

Taking  the  Sample 105 

Checking  the   Sampling    105 

Sacking  the  Samples 107 

Cutting    Down    Samples 107 

Erratic  Samples  iii 

Discussion George  E.  Collins  113 

Discussion G.  M.  Gouyard  115 

Discussion John  C.  Treadwell  115 

Discussion Benjamin  B.  Lawrence  116 

—5— 


SAMPLING   AND    ESTIMATION    OF   ORE    IN    A    MINE 

PAGE. 

Discussion Philip  Argall  1 17 

Unsystematic  Sampling George  Bancroft  118 

On  Sampling  the  Floor  of  a  Wet  Level.       Ernest  Levy  124 

Discussion Chester  F.  Lee  128 

Discussion W.  M.  Courtis  129 

Discussion Albion  S.  Howe  130 

Mine  Valuation T.  Lane  Carter  131 

Some  Aspects  of  Mine  Valuation J.  H.  Curie  138 

Discussion Walter   Harvey    Weed  145 

Discussion R.  Giltnan  Brown  148 

Discussion Richard  A.  Parker  150 

Discussion W.  L.  Austin  151 

Discussion G.  M.  Gouyard  152 

Discussion Richard  A.  Parker  154 

Discussion Blarney  Stevens  155 

Discussion Auguste  Mathez  161 

Discussion G.  A.  Denny  163 

Cost  of  Mine  Examination 170 

Sampling  Interval   172 

Sketching  Samples 172 

Recording   Samples    172 

Assay    Plans    172 

Discussion F.  H.  Minard  175 

Discussion George  A.  Packard  181 

Discussion R.  C.  Gemmell  184 

Discussion /.  Parke  Channing  192 

Discussion Forbes  Rickard  195 

Review  of  Discussion T.  A.  Rickard  199 

Index 219 


THE   SAMPLING   AND    ESTIMATION 
OF  ORE  IN  A  MINE. 

By  T.  a.  Rickard. 

introductory. 
.  It  is  held  by  some  that  to  publish  the  details  of  personal 
practice  is  unprofessional.  Others  are  of  the  opinion  that 
while  it  may  be  well  for  professional  men  to  discuss  with 
freedom,  but  among  themselves,  the  methods  they  employ 
in  their  work,  it  is  poor  policy  to  take  the  public  into  their 
confidence.  As  regards  the  first,  no  defence  should  be 
necessary  in  the  face  of  the  recent  growth  of  technical  as- 
sociations which  are  founded  for  the  avowed  purpose  of 
disseminating  the  knowledge  acquired  by  their  members ; 
while,  as  to  the  second  objection,  it  suffices  to  say  that  the 
mining  engineer  suffers  from  the  ignorance  of  the  public 
no  less  than  the  public  itself,  and,  therefore,  every  step 
which  will  promote  a  better  understanding  between  them 
is  bound  to  be  to  the  interest  of  both. 

Other  considerations,  equally  strong,  prompt  the  will- 
ing distribution  of  the  little  knowledge  which  any  indi- 
vidual among  us  may  happen  to  possess.  To  give  is  to 
receive.  No  man  realizes  the  limitations  of  his  knowl- 
edge until  he  begins  to  crystallize  it  into  writing,  and 
if  he  be  moved  by  a  fraternal  spirit  to  give  a  few  hints  to 
those  who  are  his  juniors  he  will  find  that  the  effort  will 
teach  him  more,  perhaps,  than  those  he  has  set  out  to 
help.  It  was  well  said  by  a  man  of  wide  learning  that  the 
best  way  to  find  out  all  about  a  subject  was  to  write  a 
book  upon  it. 

There  was  a  time  when  the  examination  of  a  mine  im- 
plied merely  a  perfunctory  visit  to  the  underground  work- 
ings, the  copying  of  maps  and  the  tabulation  of  the  output. 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

On  this  flimsy  foundation  it  was  considered  proper  to 
base  an  estimation  of  values.  Other  times,  other  methods. 
During  later  years  the  common  sense  of  every-day  busi- 
ness has  been  introduced  into  the  industry  of  mining  antl 
it  has  become  the  practice  to  investigate  with  a  thor- 
oughness quite  unknown  twenty  years  ago.  That  we  owe 
this  betterment  to  the  Rand  is  likely,  for,  while  those  of 
us  who  have  found  an  adequate  field  of  activity  else- 
where have  improved  our  own  methods  without  conscious 
suggestions  from  the  outside,  it  is  probable  that  the  very 
atmosphere  of  thought  breathed  by  the  earnest  men  of 
the  profession  has  been  influenced  by  the  great  develop- 
ments in  the  Transvaal  and  the  consequent  introduction 
of  a  degree  of  system  previously  rare  in  metal  mining, 
so  that  there  has  been  exerted  an  influence  not  less  val- 
uable because  it  may  not  be  directly  measurable. 

The  purpose  of  examining  a  mine  is,  usually,  to  enable 
the  engineer  to  pass  judgment  upon  the  value  of  it,  both 
present  and  prospective.  Whatever  data  he  uses  as  the 
basis  of  his  opinion  must  be  verified  as  far  as  possible. 
There  are  many  items  of  information  which,  on  account 
of  the  relations  of  time  and  place,  he  may  not  be  able  to 
test  at  first  hand,  all  the  more  reason  therefore  that  espe- 
cial care  be  taken  to  get  alongside  of  facts  in  those  mat- 
ters which  are  verifiable.  Among  these  the  three  most 
important  are  the  determination  of  the  amount  of  ore 
in  the  mine,  the  average  value  of  it,  and  the  cost  of 
turning  it  into  money. 

In  practice  these  three  determinations  are  undertaken 
in  the  reverse  order,  it  being  obvious  that  the  results  of 
sampling  will  be  meaningless  unless  you  know  how  valu- 
able the  ore  must  be  in  order  to  yield  a  profit,  and  it  is 
likewise  apparent  that  no  estimate  of  reserves  can  be 
made  without  the  safe  basis  of  one's  own  sampling  of  the 
mine. 

"Ore"  may  be  defined  as  metal-bearing  rock  which 
can  be  exploited  at  a  profit,  and  it  should  be  unnecessary 
to  distinguish  a  profit-yielding  material  as  "pay-ore" ; 
however,  it  is  a  common  habit  to  include  all  vein-matter 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

containing  any  value  under  the  term  "ore,"  it  being  left 
to  sampling  and  assaying  to  differentiate.  In  the  present 
discussion,  whenever  the  word  "ore"  is  employed,  it  will 
mean  such  material  only  as  at  the  time  of  the  examina- 
tionican  be  profitably  exploited ;  and  if  there  be  other  lode- 
stuff  which  will  yield  a  profit  in  the  event  of  a  further 
probable  reduction  of  the  expenses  of  mining  and  mill- 
ing, then  such  material  will  be  designated  as  "low-grade 
ore" ;  any  other  vein-matter,  of  less  value,  will  be  included 
under  the  general  term  "waste." 

DETERMINATION    OF    COSTS. 

In  arriving  at  the  average  costs  incurred  in  the  busi- 
ness of  a  mine,  the  engineer  will  encounter  conditions 
which  may  vary  between  the  extremes  of  a  going  con- 
cern, having  detailed  accounts  covering  a  period  suffi- 
ciently long  to  afford  thoroughly  reliable  data,  and  that 
of  an  undeveloped  mine  in  an  entirely  new  region  where 
no  such  data  are  available. 

The  first  renders  it  possible  to  obtain  the  fullest  infor- 
mation, and  in  such  instances,  if  true  figures  are  not 
secured,  it  is  often  due  to  mere  carelessness.  Sometimes 
the  costs  are  not  segregated  on  the  account  books,  and  it 
will  require  tactful  insistence  to  get  at  the  actual  facts. 
The  investigator  may  find  that  there  is  a  tendency  to 
eliminate  outlays  on  improvements  for  the  reason  that  they 
are  to  be  regarded  as  extraordinary  items  of  expenditure, 
and  it  is  not  infrequently  found  that,  in  the  case  of  a  new 
equipment,  the  heavy  item  of  wear  and  tear  is  overlooked. 
Errors  may  arise  from  the  adoption  of  figures  which 
cover  exceptional  periods;  for  instance,  at  high  altitudes 
the  costs  during  the  summer  months  are  less  than  in 
winter  on  account  of  cheaper  transport,  better  water  sup- 
ply, and  other  causes  connected  with  the  difference  of  sea- 
sons ;  similarly,  incorrect  data  as  to  mining  costs  may  be 
accepted  through  overlooking  th(i  fact  that  during  the 
period  in  question  the  amount  of  dead  work  (exploratory) 
has  been  unusually  small  in  comparison  to  that  which 
is  required  to  keep  step  with  the  stoping  operations ;  fur- 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

thermore,  in  milling  it  is  not  uncommonly  found  that 
the  power  used  during  a  part  of  the  year  is  derived  from 
water,  which  is  either  free  of  cost  or  relatively  cheap,  and 
that  during  the  remainder  of  the  year  coal  or  wood  has 
to  be  consumed,  to  generate  steam,  at  a  much  greater  ex- 
pense. These  are  some  of  the  errors  against  which  one 
has  to  be  on  guard.  Speaking  broadly,  the  safest  way 
to  avoid  them  is  to  quote  costs  covering  a  long  period, 
long  enough  to  include  the  vicissitudes  of  seasons  and 
markets. 

Mines  are  rarely  in  a  stationary  state  of  development; 
like  man  himself,  they  are  either  growing  or  declining. 
When  an  engineer  examines  a  mine  he  obtains  data  which 
are  representative  of  conditions  liable  to  change  on  ac- 
count of  the  probable  expansion  or  contraction  of  the  en- 
tire enterprise;  therefore,  the  figures  he  gets  are  apt  to 
represent  a  passing  phase  in  the  development  of  the  par- 
ticular undertaking,  and  he  cannot  arrive  at  the  average 
costs  for  the  future  by  simply  dividing  the  expenditure 
for  a  given  period  by  the  tonnage  mined  during  that 
period.  If  the  property  is  a  steady-going  concern,  which 
has  been  operated  for  many  years  under  conditions  which 
are  likely  to  continue  unchanged,  then  indeed  he  has  an 
easy  task,  and,  given  correct  figures,  he  has  only  to  per- 
form a  plain  sum  in  simple  division.  Such  cases  are  rare. 
Mines  are  usually  bought  by  capitalists  because  they  think 
they  can  enlarge  the  scheme  of  operations  so  as  to  make 
the  business  more  profitable  than  has  been  the  case  under 
the  previous  ownership.  This  often  entails  a  sweeping 
change  in  the  manner  of  carrying  on  the  enterprise;  the 
entire  business  is  conducted  on  broader  lines,  and  it 
is  assumed  that  a  larger  scale  of  operation  will  result 
finally  in  a  considerable  diminution  in  the  average  cost 
per  ton.  Apart  from  the  fundamental  supposition  of  ore- 
reserves  big  enough  to  warrant  the  expansion,  there  arises 
the  question  whether  the  money  to  be  spent  in  extra  equip- 
ment, a  better  trained  staff,  more  vigorous  and  extensive 
development  of  the  mine,  etc.,  will  result  in  a  reduction  in 
costs  to  the  extent  estimated.    Experience  has  shown  that 


SAMPLING    AND    ESTIMATION    OF    ORE   IN    A    MINE 

sanguine  expectations  in  this  regard  are  not  always  ful- 
filled, and  that  a  single  mine  owner  or  a  small  local  syndi- 
cate can  often,  in  spite  of  imperfections  of  administration 
and  equipment,  work  a  mine  at  a  cost  per  ton  which  will 
compare  favorably  with  that  of  a  big  company  having  the 
large  permanent  expenditures  inseparable  from  the  very 
nature  of  its  organization.  In  judging  of  this,  experience 
is  the  only  guide ;  no  rules  can  be  laid  down ;  each  case 
must  be  considered  apart  and  on  its  own  merits.     • 

In  the  case  of  an  undeveloped  mine,  in  a  new  district, 
reliable  data  concerning  costs  are  not  available.  Under 
such  circumstances  it  is  well  to  supplement  one's  judg- 
ment by  visiting  the  nearest  mines  which  are  being  oper- 
ated under  like  conditions.  To  those  devoid  of  experi- 
ence the  situation  is  honeycombed  with  pitfalls.  West 
Australia  afforded  many  lamentable  examples  of  this  dur- 
ing the  years  between  1895  and  1898.  The  majority  of 
reports  made  at  that  time  omitted  to  include  any  estimate 
of  costs,  all  the  available  space  being  taken  up  by  flam- 
boyant statements  of  results  obtained  from  rough  sam- 
pling, together  with  wild  prophecies  concerning  enrich- 
ment in  depth  based  upon  the  presence  of  sulphides,  tel- 
lurides,  etc.  It  is  a  fact  that  many  reports,  which  con- 
fessed !to  an  average  of  8  or  10  dwts.  of  gold  per  ton  of 
ore,  advocated  the  purchase  of  mines  situated  in  localities 
where,  at  that  time,  the  costs  could  not  possibly  be  less 
than  20  dwts.  per  ton.  Again  and  again  one  found,  in 
examining  a  mine  which  had  proved  a  failure  or  was 
about  to  collapse,  that  the  earlier  reports  contained  the 
results  of  assays  upon  what  obviously  must  have  been 
mere  specimens  of  ore.  Such  reports  had  served  as  the 
foundation  for  financiering  on  a  napoleonic  scale,  and 
prospects  had  been  highly  commended  on  the  basis  of  an 
average  tenor  low  even  under  exceptionally  favorable 
conditions,  but  quite  unattainable  in  an  uninhabited  desert 
several  hundred  miles  from  any  manufacturing  center. 

It  may  seem  that  the  follies  of  a  boom  are  hardly  worth 
castigation,  but  they  may  recur,  and  if,  haply,  they  do  not, 
then  we  may  at  least  learn  one  lesson  from  them,  namely, 

—13— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

that  it  is  quite  as  important  to  ascertain  the  average  coslis 
as  it  is  to  determine  the  average  value  of  the  ore  in  a 
mine.  To  ask  a  man  who  has  had  no  experience  in  the 
business  and  management  of  mines  to  appraise  the  value 
of  a  prospect  situated  in  a  new  region  is  to  court  disaster. 
He  may  be  a  chemist,  geologist,  mineralogist,  mechanical 
engineer ;  however  accomplished  he  may  be,  unless  he  has 
served  an  apprenticeship  as  a  mining  engineer  he  will  be 
more  helpless  than  helpful.  In  sizing  up  the  situation  it 
is  necessary  that  a  man  should  know  what  are  likely  to 
be  the  costs  of  stoping,  timbering,  road-making,  erection 
of  machinery,  equipment,  etc.,  and  these  things  he  can  only 
know  through  actual  underground  experience  and  per- 
sonal participation  in  the  administration  of  mines.  In  a 
new  district  all  the  data  obtainable  will  be  those  furnished 
by  prospectors,  diggers  and  local  promoters,  very  few  of 
whom  have  accurate  knowledge  on  these  points,  and  when 
they  have  it  they  do  not  feel  called  upon  to  donate  it  to  the 
novice  who  happens  along.  This  disregard  of  the  inevit- 
able high  costs  attendant  upon  the  opening  up  of  a  new 
mining  region  under  unfavorable  conditions  has  been  at 
the  bottom  of  the  blunders  which  have  retarded  the  early 
development  of  many  districts. 

THE  DETERMINATION  OF  THE  AVERAGE  VALUE  OF  THE  ORE. 

The  average  value  of  the  ore  in  the  past  can  be  ascer- 
tained from  the  records  of  a  mine,  but  to  find  out  the  prob- 
able average  value  of  the  production  in  the  future  there  is 
no  method  save  that  of  testing  the  ore  exposed  in  the 
workings.  This  is  done  by  taking  representative  frag- 
ments and  then  subjecting  them  to  assay.  The  method  is 
termed  "sampling."  There  are  many  ways  of  carrying 
out  the  operation;  the  best  are  the  outcome  of  experi- 
ence. 

Sampling  is  expensive.  It  cost  $7,000  to  sample  one 
well-known  mine,  and  it  cost  $12,000  to  do  the  same  work 
in  a  neighboring  property.  This  did  not  include  the  fee 
of  the  engineer  in  either  instance.  Therefore,  an  engineer 
will  not  commence  an  elaborate  sampling  of  a  large  mine 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

unless  he  has  reason  to  believe  that  the  circumstances 
warrant  it.  The  cost  moimts  up  into  big  figures,  because 
of  the  large  number  of  assistants  necessary,  the  work- 
men employed  in  rigging  up  such  timbering  as  is  required 
to  enable  the  sampling  gang  to  get  at  the  stope  faces,  the 
cost  of  assays,  and  other  expanses.  When  a  mine  has  very 
extensive  workings  the  cost  is  much  increased  on  account 
of  the  necessity  for  putting  up  special  timbering  upon 
which  to  rig  up  a  temporary  platform,  or  such  other  ar- 
rangement as  will  permit  of  convenient  access  to  the  faces 
of  ore  in  the  stopes.  The  figures  already  quoted  indicate 
very  plainly  that  a  thorough  sampling  must  not  be  lightly 
undertaken.  It  should  always  be  preceded  by  a  prelim- 
inary investigation.  In  going  through  the  mine  for  the 
first  time  the  engineer  will  observe  that  either  the  present 
value  of  it  or,  perhaps,  its  future  prospects,  hinges  upon 
certain  facts ;  it  may  be  a  question  whether  the  lower  work- 
ings exhibit  a  falling  off  in  value ;  it  may  be  important  to 
ascertain  whether  particular  ends  of  levels  or  partic- 
ular stopes  are  really  as  good  as  represented.  A  few 
samples  will  throw  light  on  these  points,  and,  if  these  are 
satisfactory,  then  it  will  be  well  to  test  certain  portions  of 
(the  mine  more  thoroughly,  and  in  this  way  finally  get 
data  which  will  determine  whether  a  complete  sampling 
of  all  the  workings  is  justified. 

In  organizing  the  sampling  gang  it  is  necessary  to  adopt 
a  system  in  order  to  avoid  confusion.  If  it  is  a  small  mine, 
or  if  the  conditions  render  it  advisable  to  keep  quite  clear 
of  any  assistance  from  the  management,  then  the  engineer 
will  employ  only  his  personal  assistants,  and,  whatever 
their  number,  he  will  divide  them  into  sets,  one  of  whom 
will  break  the  sample  while  the  other  holds  the  box  to  re- 
ceive it  as  broken.  The  accompanying  photograph  illus- 
trates such  a  couple  at  work.  If,  on  the  other  hand,  the 
examination  is  being  made  for  the  owner  of  the  mine,  or 
the  conditions  otherwise  warrant  the  engineer  in  accepting 
the  good  offices  of  the  management,  so  that  he  can  utilize 
the  services  of  workmen  on  the  property,  then  the  task  is 
easier  and  he  can  put  each  of  his  assistants  to  work  with  a 


SAMPLING   AND    ESTIMATION    OF   ORE   IN    A    MINE 

miner,  the  latter  doing  the  muscular  work  of  breaking 
the  ore  while  the  assistant  watches  to  see  that  the  sample 
is  fairly  taken,  and  collects  it  as  it  falls  into  the  box  or 
other  suitable  receptacle  made  for  this  purpose. 

Next,  it  is  necessary  to  determine  what  interval  to  allow 
between  samples.  If  the  ore  is  fairly  regular  in  width  and 
value,  an  interval  of  lo  ft.  will  usually  suffice ;  if  it  is  very 
spotty  in  value  and  subject  to  sudden  changes  in  width,  a 
lesser  interval  will  be  required.  I  have  been  compelled  to 
sample  every  3  ft.  in  the  case  of  a  vein,  the  ore  of  which 
varied  between  one  inch  and  one  foot,  with  assay-contents 
ranging  between  10  ounces  of  silver  and  1,000  ounces.  In 
such  a  case  extreme  care  to  sample  accurately  at  very 
short  intervals  is  absolutely  imperative  in  order  to  get  at 
any  kind  of  idea  regarding  the  average  of  the  ore.  When  a 
lode  is  built  up  of  the  common  sulphides,  such  as  pyrite  or 
galena,  the  variations  in  value  are  apt  to  be  small,  and  un- 
der such  conditions  20  ft.  may  not  be  unsafe.  Even  though 
you  may  decide  to  take  samples  at  much  shorter  intervals, 
it  is  best  to  start  off  with  a  large  interval,  say  of  20  ft., 
and  when  the  assay-returns  are  received  you  can  cut 
this  down  by  taking  intermediate  samples,  if  the  circum- 
stances warrant  it.  This  will  prevent  useless  labor  over 
stretches  of  poor  ground,  and  at  the  same  time  suggest 
the  proper  interval  required  wherever  the  workings  are 
in  good  ore.  Having  settled  this  point,  the  next  step  is  to 
take  the  first  assistant  and  measure  as  regularly  as  possible 
along  the  workings,  marking  each  successive  place  with 
colored  chalk  so  that  there  can  be  no  mistake  as  to  the 
point  where  the  sample  is  to  be  taken.  Then,  one  of  the 
assistants  or  the  engineer  himself,  at  all  events  a  man 
who  has  had  some  training  as  a  mine  surveyor,  is  told  off 
to  make  sketches  of  the  workings  as  the  sampling  pro- 
ceeds, noting  the  variations  in  the  vein,  the  number  of 
each  sample  and  its  position,  and  gathering  other  infor- 
mation, often  extremely  important,  such  as  develops  in 
the  course  of  the  work  done  by  the  sampling  gang.  An- 
other man  sees  to  the  correct  labeling  of  the  samples, 
the  sacking  and  sealing,   and   the   removal  of  them  to 

—16— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

a  safe  place.  All  the  data  thus  obtained  should  be  set 
down  upon  a  longitudinal  section  of  the  mine,  so  that 
they  can  be  generalized  by  the  engineer  later  on. 

THE  WORK  OF  SAMPLING. 

The  best  sampling  tool  is  a  moil  when  struck  by  a  four- 
pound  hammer.  Beware  of  the  prospecting  pick  or  the 
geologist's  hammer,  or  even  the  larger  type  of  each  which 
the  working  miner  uses;  the  former  insensibly,  but  in- 
evitably, seeks  out  the  soft  places  and  crevices  in  the  vein, 
and  does  not,  therefore,  yield  a  true  sample,  while,  as  to 
the  hammer,  that  does  the  reverse  and  tends  to  break  off 
the  projecting  points,  which  usually  represent  the  harder 
portions  of  the  ore.  As  a  rule,  the  richest  parts  of  the  vein 
are  not  in  the  hard  quartz,  but,  particularly  near  the  sur- 
face, in  the  decomposed  lode-stuff,  so  that  the  pick  gives 
too  high  an  average  and  the  hammer  one  which  is  too  low. 
By  the  right  use  of  the  moil  and  hammer  the  error  of  ex- 
tremes can  be  avoided.  Of  course,  the  excellence  of  a 
tool  depends  upon  the  right  use  of  it,  and  it  is  very  easy  to 
get  misleading  results  with  the  moil  as  with  the  other  im- 
plements already  criticized ;  nevertheless,  experience  de- 
monstrates that  the  former  is  more  likely  to  give  an  ap- 
proach to  a  perfect  sample.  The  ideal  method  of  sampling 
IS  the  testing  of  a  cheese  by  a  cheese-tester,  which  removes 
a  core  of  uniform  size.  In  a  mine  the  intention  is  to 
imitate  this  method  as  nearly  as  a  material  of  very  vari- 
able hardness  and  texture  will  allow,  an  effort  being  made 
to  cut  out  a  channeling  or  groove  of  uniform  breadth  and 
depth  across  the  full  width  of  th^  ore. 

Either  a  moil  or  a  gad  is  obviously  best  adapted  for  this 
purpose.  If  the  ore  is  too  hard  for  a  moil  and  a  single- 
hand  hammer,  get  another  striker  and  a  double-hand  ham- 
mer.^ Do  not  let  the  hardness  of  the  ore  lead  you  into  the 
mistake  of  using  dynamite,  in  the  form  of  ''pop  shots,"  in 
order  to  loosen  the  ground.  No  ground  that  can  be  mined 
in  the  ordinary  way,  that  is,  by  drilling  holes  and  charging 

^A  single-hand  hammer,  that  is,  one  requiring  the  use  of  only  one  hand, 
ordinarily  weighs  4  lbs.     A  double-hand  hammer  weighs  from  8  to  10  lbs. 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 


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FIG.    I. 


SAMPLING    AND    ESTIMATION    OF   ORE    IN    A    MINE 

them  with  an  explosive,  is  too  tough  for  a  moil  struck  by  a 
double-hand  hammer,  when  swung  by  a  good  man.  The 
use  of  an  explosive  introduces  an  element  of  danger  from 
"salting,"  as  it  is  easy  to  charge  the  cartridges  with 
powdery  gold,  which  the  dynamite  will  distribute  very 
prettily  amid  the  ore.  Of  course,  one  can  avoid  such 
tricks  by  using  one's  own  dynamite,  but  engineers,  as  a 
rule,  do  not  find  it  convenient  to  travel  about  with  high- 
grade  explosives.  Apart  from  this,  there  is  a  commoner 
danger.  Dynamite  tends  to  break  a  conical  cavity,  with 
the  drill-hole  as  the  axis  of  a  cone  which  tapers  inward. 
The  product  of  the  drill-hole  would  not  be  a  fair  sample 
even  if  it  could  all  be  secured  without  loss  or  interference. 
Usually  the  explosion  of  the  hole  breaks  a  mass  of  rock, 
the  cross-section  of  which  tapers  from  the  width  of  several 
feet  to  almost  a  point,  so  that  as  a  sample  the  material 
obtained  is  misleading.  To  pick  a  sample  out  of  a  mass 
of  ore  thus  broken,  or  to  take  it  all,  is  a  procedure  likely 
to  lead  to  serious  errors. 

An  accurate  sample  represents  a  true  cross-section  of 
the  ore ;  it  depends,  therefore,  upon  the  uniformity  of  size 
of  the  groove  or  furrow  ;  that  is  to  say,  an  equal  amount  of 
ore  must  be  broken  across  every  part  of  the  entire  width 
of  the  lode.  That  is  what  makes  sampling  difficult,  espe- 
cially in  gold-veins,  the  predominant  matrix  of  which  is 
quartz,  in  some  form,  varied  by  softer,  more  friable  min- 
erals, which  cause  marked  contrasts  in  the  ease  of  frac- 
ture. In  one  case,  which  came  under  my  notice,  it  took  six 
men  (three  of  whom  moiled  while  the  other  three  held  the 
boxes  to  receive  the  samples)  the  whole  of  one  shift  to 
take  three  samples  across  a  vein  12  ft.  in  cross-section, 
and  in  accomplishing  this  they  dulled  35  moils.  That  was 
good,  honest  sampling. 

THE  SIZE  OF  THE  SAMPLE. 

This  question  is  an  important  one.  It  is  a  matter  to 
be  decided  by  convenience,  scientific  principle  and  experi- 
ence. We  will  consider  the  least  important  first.  Large 
samples  are  more  difficult  to  handle  than  small  ones  and 


SAMPLING   AND    ESTIMATION    OF   ORE    IN    A    MINE 

require  more  assistance.  In  mountainous  regions  or  in 
desert  places,  where  facilities  for  crushing  the  ore  are 
lacking,  it  will  be  found  inconvenient  to  break  samples  so 
big  that  their  reduction  by  hand  consumes  much  time. 
Occasionally  greater  inconvenience  than  this  will  arise 
through  the  want  of  facilities  for  removing  the  ore  from 
the  mine  without  entrusting  it  to  unsafe  hands.  The 
factor  of  time  has  been  mentioned ;  that  of  cost  must  not  be 
forgotten,  for  time  and  money  are  valuable  alike  to  the 
engineer  and  to  his  client,  so  that  the  former  will  find  it 
advisable  "to  cut  the  coat  according  to  the  cloth."  Every 
engineer  runs  up  against  obstacles  such  as  have  been  re- 
ferred to,  and  these  vary  so  much  with  each  individual 
case  that  it  is  needless  to  attempt  to  specify  them  in  greater 
detail. 

The  size  of  the  sample  will  also  depend  upon  the  facility 
with  which  the  ore  can  be  broken,  because  the  scientific 
principle  underlying  the  act  of  sampling  is  the  obtaining 
of  a  true  cross-section  of  the  lode.  In  the  case  of  an  ore 
having  the  consistency  of  cheese,  which  is  by  no  means  an 
impossible  occurrence  and  is  approached  by  certain  lodes 
which  are  built  up  of  crushed  material,  a  perfect  sample 
can  be  obtained  by  running  a  "scraper"  over  it  so  as  to 
make  a  narrow  furrow  across  the  full  width  of  it.  This 
will  result  in  a  sample  of  minimum  size.  On  the  other 
hand,  in  order  to  obtain  a  true  sample  of  a  large  lode  made 
up  of  streaks  of  varying  hardness  and  uneven  fracture  it 
will  be  found  necessary  to  break  a  hundred  pounds  at  the 
very  least.  In  such  a  case  it  is  impracticable  to  secure  a 
sample  of  the  hardest  portions  except  in  large  irregular 
pieces,  and  this  necessitates  the  breaking  of  a  propor- 
tionate amount  of  material  from  those  places  w^here  the  ore 
is  much  softer  or  of  easier  fracture.  In  hard  quartz-veins 
it  will  be  found  that  a  channeling  from  4  to  6  ins.  in  width 
and  from  >^  to  i  in.  deep  will  be  adapted  to  the  securing 
of  a  true  sample ;  in  those  instances  where  the  ore  has  a 
fairly  even  grain,  as  with  replacement  deposits  in  igneous 
rocks  of  granular  texture,  it  will  be  found  that  a  groove  3 
ins.  wide  and  from  ^  to  ^  in.  deep  will  give  a  true  sam- 


SAMPLING   AND    ESTIMATION    OF    ORE    IN    A   MINE 

pie.  The  minimum  size  of  groove  which  will  yield  a  cor- 
rect result  should  be  chosen  because  any  unnecessarily 
large  groove  simply  increases  all  the  work  of  subsequent 
reduction  and  handling  of  the  samples  without  a  commen- 
surate increase  of  accuracy  in  the  final  results.  Further- 
more, one  has  always  to  remember  that  two  samples  of 
50  lbs.  each  at  5  ft.  apart  are  better  than  one  sample  of 
100  lbs.  at  an  interval  of  ^10  ft.  The  whole  idea  under- 
lying the  operation  is  that  of  securing  an  average,  and  it  is 
obvious  that  the  larger  the  number  of  data  the  more  likely 
one  is  to  approximate  the  truth. 

The  next  factor  is  more  important  than  the  two  already 
considered.  I  refer  to  that  invaluable  guide.  Experience, 
without  whom  all  work  of  this  kind  is  as  dangerous  as  is 
mountaineering  in  the  Alps  to  a  thoughtless  tenderfoot. 
To  one  who  has  done  his  own  sampling  and  assaying  there 
has  often  arisen  the  inevitable  contrast  between  the  di- 
mensions of  the  sample  and  those  of  the  gold  button  which 
represents  its  contents.  Whatever  the  size  of  the  original 
sample,  the  outcome  is  merely  one  button  of  minute  size. 
Gold,  as  at  present  known  to  exist  in  workable  lodes, 
occurs  in  a  metallic  form  and  is  usually  disseminated 
through  the  ore  in  an  extremely  irregular  and  sporadic 
manner.  As  a  consequence,  the  final  pulp  taken  for  assay, 
and  weighing,  as  a  rule,  about  an  ounce,  if  not  less,  is  apt 
to  contain  a  coarse  speck  of  gold,  the  accidental  presence 
of  which  vitiates  the  result.  Whatever  the  weight  of  the 
original  sample,  whether  5  lbs.  or  500  lbs.,  the  particular 
particle  of  gold  included  within  the  final  pulp  will  have 
the  same  effect  of  exaggeration,  save  in  one  respect,  viz., 
that,  given  the  fact  of  its  occurrence  in  the  original  sam- 
ple, it  is  more  likely  to  find  its  way  into  the  assay  pulp  of 
a  small  sample,  the  latter  being  to  the  former  in  the  pro- 
portion of  I  oz.  to  5  lbs.,  or,  say,  i  to  80,  than  in  a  large 
sample  the  final  pulp  of  which  represents,  say,  one  part  in 
8,000  parts  of  the  original.  When,  however,  the  jjold  is 
not  present  in  the  usual  condition,  but  occurs  in  that  pul- 
verulent state  known  as  "mustard  gold,"  characteristic 
of  the  metal  when  it  is  the  product  of  decomposed  tellu- 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

rides,  then  the  more  even  dissemination  ol  the  gold  causes 
it  to  be  spread  throughout  the  sample  so  as  to  make  the 
size  of  the  latter  a  factor  of  safety.  Of  course,  usually,  the 
particles  of  gold  become  flattened  out  during  the  process 
of  crushing  (on  the  buck-board)  previous  to  the  assay,  so 
as  to  appear  on  the  screen  in  the  form  of  scales,  termed 
^'metalHcs,"  and  the  practice  is  to  pick  them  up,  weigh 
them,  cupel  them,  and  then  determine  their  weight  in  re- 
lation to  the  weight  of  the  sample ;  but  this  does  not  over- 
come the  interference  with  accuracy  because,  although 
this  determines  the  proportions  in  which  such  particles  oc- 
curred in  the  particular  sample,  it  does  not  give  any  clue 
as  to  the  relative  importance  of  such  particles  in  the 
enrichment  of  the  entire  lode.  Naturally,  in  small  sam- 
ples the  interference  is  relatively  greater  and  therefore 
more  clearly  recognizable.  On  the  whole,  therefore,  one 
comes  back  to  the  conclusion  that  the  best  rule  to  follow 
is  the  taking  of  the  smallest  sample  consistent  with  se- 
curing a  true  average  of  the  lode  at  each  cross-section. 
The  larger  the  sample,  the  more  difficulty  in  handling  it, 
the  more  persons  required  to  help  and  the  greater  the 
chances  of  poor  work.  Let  me  mention  an  exam- 
ple. Two  engineers  examine  a  mine,  and,  in  carry- 
ing out  their  investigations,  one  gets  large  samples,  re- 
sulting from  a  wide  groove,  while  the  other  takes  small 
samples,  the  product  of  a  smaller  groove.  Although  both 
samples  are  equally  good,  in  so  far  as  they  represent  an 
approach  to  the  true  cross-section  of  the  lode  at  each  place 
sampled,  nevertheless  the  former,  on  account  of  the 
greater  size  of  the  samples,  is,  theoretically,  the  better  of 
the  two.  However,  the  second  engineer  employed  fewer 
assistants,  and  all  those  whom  he  employed  in  this  capac- 
ity were  men  whose  antecedents  he  knew  and  whose  re- 
liability he  had  previously  tested,  while  the  first  engineer 
engaged  his  gang  of  samplers  at  the  mine,  most  of  them 
being  vouched  for  by  the  management  or  by  a  fellow- 
engineer  ;  yet  the  chances  of  error  were  increased  by  the 
number  of  men  employed,  the  real  ability  of  each  to  take 
true  samples  being  merely  assumed  on  the  statement  of 
some  one  else. 


SA.VPIJXr,    AND    ESTIMATION    OF    ORE    IX    A    MINE 


THE  REDUCTION  OF  THE  SAMPLES. 

When  the  samples  are  broken  they  are  put  into  sacks 
which  are  not  marked  upon  the  outside,  but  are,  prefer- 
ably, labeled  by  inserting  a  tag  with  the  number  upon  it. 
This  tag  is  often  merely  a  piece  of  paper,  detached  from  a 
notebook,  but  in  this  form  it  is  apt  to  get  torn  or  the 
number  upon  it  obscured,  especially  when  the  ore  is  moist, 
therefore  it  is  best  to  use  a  metal  or  wooden  tag  especially 
prepared  for  this  purpose.  The  latter  will  be  found  con- 
venient. Get  a  lot  of  small  pieces  of  soft  wood  (ys  in. 
thick,  I  in.  wide  and  i^  ins.  long),  and  mark  the  num- 
bers of  the  samples  upon  them  by  the  use  of  a  hard  pencil ; 
this  will  remain  as  a  visible  indentation  even  after  the 
pencil  trace  has  been  rubbed  off. 

The  samples  are  then  removed  to  a  safe  place,  either 
temporarily  in  the  mine  itself  or  to  a  building  where  they 
can  be  locked  up.  Then  comes  the  work  of  reducing 
them  in  bulk  by  crushing  and  subdivision.  If  an  assay 
office  is  conveniently  at  the  engineer's  disposal,  he  will 
probably  find  a  rock-breaker  which  he  can  use,  otherwise 
a  portable  rock-breaker,  worked  by  hand,  will  be  found  a 
useful  machine  to  take  with  him  when  a  large  sampling 
job  is  to  be  done.  In  the  absence  of  these  conveniences  the 
ore  is  broken  by  hand  with  a  cobbing  hammer,  to  the  size 
of  walnuts,  and  then  subdivided.  This  is  followed  by  fur- 
ther reduction  in  size  and  subsequent  subdivision  by 
quartering. 

It  is  usual  to  place  the  crushed  ore  upon  a  square  sheet 
of  canvas,  which  is  rolled  backward  and  forward  in  op- 
posite directions  in  such  a  manner  as  to  mix  the  ore  lying 
upon  it,  until  finally  a  conical  pile  is  left  standing  in  the 
center.  This  is  flattened  to  a  frustum  previous  to  quarter- 
ing, the  two  opposite  quarters  being  taken  and  again 
mixed  previous  to  a  further  mixing  and  quartering,  until 
the  bulk  of  the  sample  has  been  reduced  to  the  size  con- 
sidered suitable  for  shipment  to  the  assay  office. 

The  foregoing  method  is  open  to  criticism.  In  the  first 
place,  the  rolling  canvas  is  not  nearly  so  good  a  way  of 


SAMPLING   AND    ESTIMATION    OF    ORE   IN    A    MINE 

mixing  as  it  looks,  the  fines  are  apt  to  slide  over  the  sur- 
face of  the  canvas  instead  of  becoming  thoroughly  min- 
gled with  the  coarser  particles ;  moreover,  the  cone  which 
is  finally  formed  is  deceptive  in  that  the  fines  are  likely  to 
be  collected  not  at  the  center  of  the  base  of  the  cone,  as 
is  supposed,  but  to  one  side,  so  that,  in  quartering,  any 
particular  division  may  include  an  undue  proportion  of  the 
fines,  which  usually  form  the  richest  part  of  the  sample. 
Further,  in  flattening  the  cone  into  a  frustum,  for  con- 
venience in  quartering,  it  is  difficult  to  distribute  the  ore 
evenly,  and  though  great  care  be  taken  to  draw  the  ore  in 
a  straight  line  outward,  toward  the  circumference,  the 
distribution  is  liable  to  be  faulty,  and  this  part  of  the 
work  may  be  so  imperfectly  done  as  to  become  a  source  of 
error. 

An  alternative,  and  better,  method  can  be  suggested. 
Get  a  few  short  boards,  or  cause  some  to  be  sawed  to  the 
required  length,  of  about  6  ft.,  and  put  them  together  so 
that  they  will  make  a  platform  which  can  be  kept  firmly  in 
place  by  being  spread  upon  a  couple  of  sills  and  wedged  in 
with  stones.  Then,  if  the  joints  are  not  tight,  put  your 
sheet  of  canvas  upon  it,  not  to  roll  the  samples  within  it, 
but  merely  to  prevent  any  leakage  of  fines  through  the 
cracks  between  the  boards.  When  the  sample  is  crushed, 
gatfici  it  up  with  a  scoop  or  other  handy  implement  to  the 
center,  lifting  the  broken  ore,  shovelful  by  shovelful,  and 
pouring  it  as  nearly  as  possible  at  the  same  central  point 
so  as  to  aid  the  mixing  of  it.  In  doing  this  it  will  be  found 
convenient  to  use  the  "cone,"  in  vogue  at  many  smelters, 
which  consists  of  a  sharp  central  cone,  made  of  iron, 
with  four  thin  radiating  partitions  which  cause  the  ore,  as 
it  falls  upon  the  center,  to  become  quartered. 

In  order  to  do  accurate  work  at  this  stage,  it  is  neces- 
sary that  the  particles  of  ore  should  not  vary  too  much  in 
size.  The  fines  are  apt  to  obscure  the  fact  that  there  are  a 
good  many  large  lumps,  and  the  unaided  eye  is  likely  to 
mislead  in  this  respect.  For  this  reason  it  is  well,  if  con- 
venient, to  use  a  wire  screen,  say,  Yz  or  ^-inch  mesh,  or 
perhaps  two,  one  of  %  and  the  other  of  Y^-mch  mesh, 

—24— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

to  be  employed  at  successive  stages  of  the  operation,  so 
that  the  maximum  size  of  the  particles  can  be  kept  within 
defined  limits.  It  is  an  easy  matter  to  take  a  piece  of  wire 
cloth,  say  i  foot  square,  and  have  a  frame  put  around  it 
when  you  reach  the  mine. 

If  it  is  necessary  to  send  the  samples  to  a  distant  assay 
office  or  to  take  them  with  you  on  your  departure  from  the 
mine,  then  it  becomes  convenient  to  reduce  them  until  they 
weigh  only  three  or  four  ounces  each.  In  doing  this  the 
engineer  will  anticipate  the  work  of  further  reduction, 
which  is  usually  carried  out  by  the  assayer.  The  samples 
will  be  crushed  smaller  and  passed  through,  say,  a  lo-mesh 
screen,  and,  instead  of  quartering,  it  will  be  well  at  this 
stage  to  use  a  gridiron  sampling  device,  which  consists  of 
a  series  of  metallic  scoops  separated  by  vacant  spaces  of 
equal  width,  so  that  one-half  of  the  ore  falls  through  while 
the  remainder  is  arrested.  When  this  method  has  reduced 
the  bulk  of  the  samples  to  the  desired  dimensions,  they  are 
put  into  small  paper  or  canvas  sacks,  the  latter  preferably, 
especially  if  it  is  intended  to  ship  them  a  long  distance. 

If  genius  be,  as  has  been  authoritatively  stated,  "an 
infinite  capacity  for  taking  pains,"  then  it  is  safe  to  say 
that  genius  is  exactly  the  mental  quality  needed  for  the 
humdrum  work  of  sampling,  for  to  do  it  conscientiously 
and  well  requires  patience,  strength  and  an  amount  of  un- 
wearied watchfulness  sufficient  to  elevate  this  common 
task  to  the  level  of  a  fine  achievement.  It  requires  an 
obstinate  persistence  to  get  a  true  average  across  a  hard 
and  tough  quartz-vein ;  any  relaxation  of  care  or  muscle 
will  at  once  result  in  the  spoiling  of  the  sample  and  the 
consequent  introduction  of  an  error  into  the  calculations  of 
the  engineer.  It  needs  judgment  to  know  how  to  treat  a 
cavity  (or  vug)  or  an  unusual  inclusion  of  waste  rock; 
it  needs  a  nice  sense  of  proportion  to  avoid  cross-sections 
which  are  exceptional,  to  break  an  equal  weight  of  ore 
along  a  line  lo  or  12  feet  in  length,  and  to  get  the  true 
width  of  an  irregular  cutting.  For  these  reasons  it  is  best, 
when  carrying  out  an  arduous  scheme  of  sampling,  to 

—25— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

divide  the  muscular  from  the  mental  work,  allowing  a 
miner  to  do  the  actual  breaking  under  the  direction  of  an 
intelligent  trained  assistant,  who  holds  the  receptacle  for 
the  ore  as  it  falls,  and  at  the  same  time  watches  the  move- 
ments of  the  miner.  Further,  it  is  well  to  make  the  hours 
of  labor  short,  so  as  to  avoid  an  excessive  strain  on  the 
faculties,  such  as  will  cause  relaxation  of  the  intent  watch- 
fulness and  care  necessary  to  good  sampling.  In  order  to 
escape  the  risk  of  inferior  work,  it  is  good  practice  to  vary 
it,  as,  for  instance,  by  putting  the  assistants  to  surveying 
or  mapping  for  a  day  or  so,  at  intervals.  Otherwise  your 
men  are  apt  to  get  "stale"  through  weariness.  How  tire- 
some such  work  is  those  can  testify  who  have  done  much 
of  it ;  the  dirt,  the  wet,  the  strained  positions,  the  splinters 
that  hit  the  face  and  hands,  the  obstinacy  of  rock  and  cir- 
cumstance, the  weary  iteration  of  it — these  require  some- 
thing better  than  mere  mule-like  persistence  to  overcome 
them,  and  the  man  who  can  do  a  difficult  piece  of  sampling 
honestly  and  well  can  be  entrusted  to  do  work  for  which 
much  greater  credit  is  usually  given  by  those  in  authority. 

PRECAUTIONS   IN    SAMPLING. 

Although  the  greater  thoroughness  with  which  mines 
are  investigated  nowadays  has  made  trickery  scarce,  in- 
stances of  the  latter  do  occur  occasionally.  They  are 
rarely  exposed  because  of  the  lack  of  evidence,  and  there- 
fore the  occurrence  of  them  is  obscured  amid  those  fail- 
ures and  disappointments  in  mining  which  arise  from 
other  causes.  The  tampering  with  samples,  called  'salt- 
ing,' and  the  blocking  up  of  workings  which  might  give 
unfavorable  testimony  regarding  the  condition  of  a  mine 
are  two  possibilities  against  w^hich  one  must  be  continually 
on  guard.  To  prevent  'salting'  it  is  imperative  that  the 
work  be  done  by  trustworthy  assistants  ;  and  in  the  case  of 
a  large  mine,  where  it  becomes  necessary  to  employ  work- 
men whose  antecedents  are  unknown,  it  is  well  to  arrange 
that  the  w^ork  be  done  in  pairs,  the  miner  breaking  the 
samples  under  the  direction  of  an  assistant,  who  holds  the 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

box  to  catch  the  sample  as  it  is  broken.  When  the 
sampHng  is  done  it  is  well  for  the  chief  himself  to  take  a 
certain  number  of  samples,  aided  by  his  first  assistant, 
these  samples  being  taken,  not  at  haphazard,  but  in  such 
a  way  as  to  check  the  previous  work.  One  of  the  best 
guards  against  any  successful  tampering  with  one's 
samples  is  to  take  an  occasional  sample  of  waste.  If  the 
samples  are  all  salted,  the  assay  of  the  waste  will  disclose 
the  fact.  Occasionally  it  may  be  well  to  fill  one  or  two 
sacks  with  material  the  exact  assay  contents  of  which 
have  been  previously  determined.  In  any  event,  it  is  bet- 
ter not  to  use  sacks  which  are  numbered  or  otherwise 
marked  on  the  outside,  because,  should  trickery  be  pur- 
posed, such  marks  make  it  easy  to  note  from  what  parts 
of  the  mine  the  various  samples  come  and  to  'salt'  them 
accordingly.  It  is  well  to  assay  the  samples  on  the  spot, 
if  a  suitable  assay  office  is  available,  particularly  when  the 
engineer,  or  one  of  his  assistants,  is  a  good  assayer,  as  is 
frequently  the  case.  Of  course,  in  using  a  strange  assay 
plant  it  is  necessary  to  guard  against  fraud,  and  to  this 
end  it  is  well  to  test  the  fluxes  used  by  assaying  a  charge 
without  ore  every  time  a  batch  of  samples  is  put  through 
the  furnace. 

During  the  interval  which  elapses  between  the  time 
when  the  sample  is  first  broken  in  the  mine  and  its  final 
assay,  it  is  necessary  that  the  sack  containing  it  should  be 
sealed.  It  is  a  good  thing  to  use  uncommon  wax  and  an 
imcommon  seal,  so  as  to  render  trickery  more  difficult. 
It  is  well  even  to  use  a  peculiar  kind  of  string  for  tying  up 
the  sacks.  "He  is  most  free  from  danger  who,  even  when 
safe,  is  on  his  guard."  Any  extra  precaution  should  never 
be  considered  a  nuisance;  on  the  contrary,  it  ought  to 
become  a  habit.  Cases  of  'salting'  have  been  known  where 
the  ore  has  been  artificially  enriched  without  breaking  the 
seal  and  without  puncturing  the  sack,  and  I  know  of  an 
instance  in  which  samples  of  copper  ore,  put  into  a  carpet 
bag,  were  withdrawn  and  substituted  with  others  by  re- 
moving the  bottom  of  the  bag  and  sewing  it  up  again  while 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

the  engineer  was  asleep.  "Dead  things  will  crawl." 
"Eternal  vigilance  is  the  price  of  safety." 

Unfortunate  consequences  have  sometimes  ensued  from 
the  failure  of  an  engineer  to  see  all  the  workings  of  a 
mine.  This  may  be  due  either  to  carelessness  or  over- 
sight, but  it  may  also  be  due  to  the  rascality  of  the  mine 
owner.  Cross-cuts  are  sometimes  blocked  up  with  old 
timbers,  drifts  may  be  allowed  to  cave,  shafts  may  be 
under  water;  in  each  of  these  cases  the  engineer  must 
realize  that  he  is  under  a  responsibility  if  he  passes  judg- 
ment on  the  mine  without  seeing  for  himself  what  these 
inaccessible  workings  have  to  tell.  Their  testimony  may 
be  unfavorable;  it  usually  is  in  such  cases;  but,  on  the 
other  hand,  there  may  be  circumstances  which  influence 
the  owners  in  desiring  to  temporarily  depreciate  their 
property. 

An  opportunity  for  splendid  business  was  lost  in  the 
case  of  the  great  Broken  Hill  mine  through  an  error  of 
this  kind.  An  engineer,  and  a  good  one,  too,  was  engaged 
by  a  Melbourne  financier  to  make  an  examination  of  the 
new  discovery  at  Broken  Hill,  with  a  view  to  the  purchase 
of  an  interest.  He  duly  reached  the  mine,  and  found  a 
gang  of  miners  engaged  in  sinking  a  prospect  shaft,  which 
at  that  time,  in  1885,  was  about  70  feet  deep.  Much  to 
his  annoyance,  he  was  refused  permission  to  go  under- 
ground, except  by  written  order  from  the  manager,  who 
had  left  the  day  previous  for  Adelaide.  Disappointed,  but 
not  without  hope  of  getting  information,  he  chatted  with 
the  men,  more  especially  the  foreman,  and  endeavored  to 
pump  the  real  facts  out  of  them.  Their  talk  indicated  that 
no  rich  ore  had  been  found,  and  that  the  prospects  were 
poor.  He  examined  the  dump,  took  samples  of  it,  and 
finally  returned  to  Melbourne  via  Adelaide.  The  samples 
from  the  dump  gave  16  ounces  of  silver  at  the  best.  He 
advised  his  client  to  keep  out.  A  few  days  afterwards  it 
became  known  that  a  rich  mine  had  been  found  at  Broken 
Hill.  He  had  been  fooled,  the  discovery  having  been 
made  just  previous  to  his  visit  and  covered  up  for  a 

—28— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

particular  purpose.  That  mine  has  since  produced  over 
100,000,000  ounces  of  silver. 

This  is  one  side  of  the  question.  It  is  rarely  that  a 
bonanza  is  kept  out  of  sight.  As  a  rule,  the  exclusion  of 
an  engineer  from  certain  parts  of  a  mine  is  intended  to 
cover  unfavorable  testimony.  It  is  therefore  of  the  great- 
est importance,  more  especially  in  a  small  mine,  the  char- 
acter of  which  has  not  been  truly  established,  that  an 
effort  be  made  to  personally  investigate  all  the  v^^orkings. 
Intentional  deception  is,  I  am  glad  to  believe,  rare ;  never- 
theless, in  passing  upon  the  purchase  of  property,  the 
engineer  should  write  across  his  notebook,  ''Caveat  emp- 
tor/' One  instance  will  suffice.  Let  the  accompanying 
section.  Fig.  2,  represent  the  workings  of  a  small  mine, 
where  the  level  A  D  is  200  ft.  from  surface  and  F  M  is 
100  ft.  deeper.  Above  A  D  there  has  been  a  line  of  stopes 
from  B  to  C,  a  distance  of  200  ft.,  all  the  ground  being 
worked  out,  with  results  testified  to  by  certified  returns 
from  n^ine  and  smelter.  When  the  mine  is  sampled,  it  is 
found  that  there  is  good  ore  in  the  floor  of  level  A  D,  and 
along  the  back  of  the  lower  one  F  M,  as  indicated  in  the 
section.  Moreover,  the  raise  at  H  is  going  up  in  good  ore, 
and  the  drift  (at  M)  is  proceeding  in  ore  of  an  average 
tenor;  in  short,  the  evidence  proves  that  the  ore-body  is 
persisting  downward  with  a  pitch  to  the  east,  similar  to 
its  behavior  in  the  upper  workings.  No  winze  has  been 
sunk  below  AD.  At  E  there  is  a  hole  about  5  ft.  deep, 
which,  according  to  the  statement  of  the  superintendent, 
is  being  used  as  a  sump  to  catch  the  drippings  from  the 
stopes  and  thus  prevent  the  water  running  down  the  shaft. 
A  small  pump  at  A  sends  this  water  to  surface. 

All  looks  serene  and  straightforward,  but  the  facts  are 
as  shown  in  Fig.  3.  At  E  there  is  a  vertical  winze  which 
is  deeper  than  5  ft.,  and  has  been  carefully  filled  up.  From 
the  bottom  of  this  (at  P)  an  incline  has  been  run  down  to 
N ;  at  N  there  is  a  drive  R  O,  and  all  the  ground  above 
has  been  stoped  out  bodily,  leaving  a  mere  shell  under 
the  level  between  B  and  C.    The  raise  H  is  situated  so  as 

—29— 


SAMPLING    AND    ESTIMATION    OF   ORE    IN    A    MINE 


Hidden  Stopes 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

to  miss  these  secret  workings.  The  heart  of  the  ore-body 
has  been  taken  out,  and  a  clever  piece  of  trickery  has  been 
attempted. 

As  a  possible  check  against  the  perpetration  of  such 
practical  jokes,  it  is  not  out  of  place  to  ask  the  manager 
in  charge  of  the  mine  to  sign  z  statement  which  sets  forth 
that  he  has  informed  the  engineer  of  all  the  existing  work- 
ings, and  to  this  can  be  attached  a  map  or  a  brief  descrip- 
tion of  such  workings  as  are  inaccessible  through  caving 
or  other  causes.  Such  a  paper  will  serve  as  a  record  to 
make  clear  the  position  of  the  engineer  and  fix  the  respon- 
sibility on  the  management  of  the  mine  should  false  state- 
ments have  been  made  with  the  intent  to  deceive. 


WRONG  METHODS  OF  SAMPLING. 

In  the  early  days  of  Western  Australia,  and,  indeed,  one 
may  say  in  the  early  days  of  most  gold-fields  almost  any- 
where, it  was  a  common  practice  for  the  gentlemen, 
vaguely  known  to  the  press  as  "experts,"  to  sample  an 
incline  shaft,  on  a  vein,  by  having  a  few  shots  put  into  the 
ore,  and  then  collecting  the  material  thus  thrown  down 
to  the  bottom  of  the  shaft  and  having  it  hoisted  to  sur- 
face, where  it  formed  one  of  those  "large"  samples,  the 
tonnage  of  which  was  referred  to  with  pride  in  the  re- 
ports as  evincing  an  accurate  testing  of  the  value  of  the 
ore  in  the  mine.  Such  work  is  the  travesty  of  sampling. 
A  bunch  of  specimen  ore,  a  few  inches  in  extent,  was 
enough  to  vitiate  the  whole  result.  Such  spots  of  free 
gold  were  commonly  characteristic  of  the  Westralian  reefs 
near  the  surface,  and,  if  this  unintentional  "salting"  was 
not  enough,  the  subsequent  performance  at  surface  and 
the  handling  of  the  ore  by  a  large  number  of  men  of  un- 
known character  gave  sufficient  opportunity  for  further 
tampering  with  this  suppositious  sample.  There  is  a  lot 
of  this  sort  of  thing  perpetrated  during  the  windy  days  of 
mining  booms,  not  in  Western  Australia  alone.  : 

In  the  case  of  wide  lodes — that  is,  such  as  exceed  th'e 
average  stoping  width—it  is  advisable  to  take  sectional 

—31— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 


samples,  dividing  the  lode  into  successive  divisions,  each  of 
which  is,  say,  4  ft.  across.  The  results  thus  obtained  will 
be  useful  in  indicating  the  distribution  of  values.  Occa- 
sionally it  will  be  discovered  that  a  vein  is  being  worked 
for  a  bigger  width  than  circumstances  justify,  while,  quite 
as  commonly,  it  may  be  found  advisable  to  change  the 
practice  radically,  and  in  the  opposite  way ;  that  is,  it  may 
be  proved  by  sectional  sampling  that,  while  a  mine  cannot 
be  profitably  operated  if  only  a  narrow  width  of  rich  ore 
is  mined,  it  will  become  remunerative  if  it  is  worked  on  a 
larger  scale  by  adopting  a  bigger  stoping  width,  so  as  to 
include  parallel  streaks,  feeders  and  branch  veins,  which 
will  yield  a  much  bigger  tonnage  of  low-grade  ore,  unsuit- 


'^i?^^ 


Fig.  4 


able,  it  may  be,  for  shipment,  but  profitable  in  a  mill,  to  be 
erected  at  the  mine  or  near  it. 

In  the  sectional  sampling  of  wide  lodes  particular  care 
must  be  taken  to  get  the  true  width.  Measurements  must 
be  taken  at  right  angles  to  the  walls  of  the  vein.  In 
sampling,  however,  a  horizontal  line  may  be  followed,  if  it 
is  convenient ;  it  will  give  a  larger  quantity  of  ore  than  a 
right-angle  section,  but  if  each  sample  is  taken  along  a 
parallel  line,  the  proportion  will  be  maintained  and  a  true 
sample*  will  be  secured. 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

The  importance  of  an  accurate  recognition  of  the  slope 
of  a  vein  and  the  pitch  of  an  ore-shoot,  when  making  esti- 
mates, cannot  be  emphasized  too  much.  An  example 
will  be  of  service.  Thus,  in  Fig.  4,  A  B  and  D  F  repre- 
sent an  ore-shoot  traversed  by  the  two  levels,  which  are 
100  ft.  apart.  The  ore  has  been  stoped  out  above  the 
upper  level,  and  a  portion  had  been  removed  between 
the  levels.  The  stopes — inaccessible — above  B  C  and 
the  stoping  begun  at  the  bottom  of  the  left-hand  raise 
are  calculated  to  obscure  the  real  condition  of  affairs. 
The  engineer  samples  the  raise  E  B  and  the  back  of  the 
level  between  E  and  F,  with  results  which  cause  him 
to  assume  a  block  of  ore  A  E  F  C.  He  has  failed  to 
recognize  the  pitch  of  the  ore-shoot  because  the  stoping 
above  B  C  has  obscured  it.  It  will  be  said  that  if  he 
had  sampled  the  bottom  of  the  level  this  mistake  might 
not  have  occurred,  but  the  sampling  of  bottoms  is  not 
always  practicable,  and  is  usually  very  unsatisfactory 
on  account  of  water  and  other  factors.  It  is  not  realized 
often  enough  that  ground  which  has  been  stoped  was  not 
necessarily  profitable.  Stopes  are  frequently  started  with 
the  hope  of  an  improvement  or  with  the  purpose  of  test- 
ing a  run  of  ground.  For  this  reason  the  workings  of  a 
mine,  both  underground  and  on  the  map,  are  apt  to  sug- 
gest unwarranted  deductions  as  to  the  distribution  of  ore, 
and  many  misleading  inferences  have  been  caused  thereby. 
Thorough  sampling  will  usually  make  the  truth  clear  to  an 
experienced  man. 

Among  the  things  to  be  avoided  one  must  mention  the 
so-called  "grab"  sample.  This  is  the  last  resort  of  inca- 
pacity. A  grab  or  haphazard  handful  of  ore  is  taken  in- 
discriminately from  all  over  a  pile  of  ore  at  the  face  of  a 
level  or  in  the  stopes,  and  this  is  put  into  a  small  sack  for 
subsequent  assay.  The  idea  of  the  grab  sample  is  to  shut 
your  eyes  and  be  "absolutely  impartial,"  but  the  brutal 
fact  is  that  one  usually  gets  a  deceptive  proportion  of  the 
fines  and  quite  disregards  the  large  pieces  of  waste  or  poor 
rock  scattered  through  the  heap.    It  is  still  the  practice  in 

—33— 


SAMPLING    AND    ESTIMATION    OF    ORE   IN    A   MINE 

•many  mines  for  the  foreman  to  take  samples  in  this  man- 
ner while  making  his  daily  round  of  the  workings.  As  a 
consequence,  the  record  of  the  assay  office  is  often  an 
iridescent  dream,  which  may  mislead  the  management 
and  become  the  cause  of  a  serious  error  in  the  estimates 
of  ore.  It  takes  more  time  to  use  a  moil  and  a  hammer, 
thereby  obtaining  a  true  average  sample,  than  it  does  to 
pick  up  a  "grab"  and  stick  it  into  one's  pocket  or  into  a 
sack;  for  this  reason  the  former  procedure  is  objected  to 
by  many  foremen.  The  fact  is,  the  daily  sampling  of  the 
faces  of  ore  should  be  a  task  allotted  to  men  who  have  the 
lime  and  the  training  for  such  work;  it  should  be  no  part 
•of  the  duty  of  a  foreman  or  a  shift-boss,  both  of  whom 
are  usually  men  of  a  type  which  confounds  system  with 
"'red  tape,"  but  it  should  be  placed  in  the  surveyor's  de- 
partment, so  that  the  record  of  results  can  be  incorporated 
with  the  maps  of  the  mine.  ''Chaciin  a  son  metier  et  les 
^'aches  seront  hiengardeesy  It  is  a  good  saying.  Let  the 
important  work  of  sampling  be  put  into  proper  hands, 
separate  from  the  particular  supervision  of  the  operations 
of  the  mine,  the  control  of  the  men  and  other  departments 
with  which  it  has  no  kind  of  connection.  The  ordinary 
multifarious  official  has  no  time  for  a  job  which  essentially 
requires  time  to  do  it  properly.  And  before  everything 
else,  begin  by  prohibiting  "g^ab  samples"  in  any  form ! 


CALCULATIONS    AFTER    SAMPLING. 

The  calculations  consequent  upon  sampling  are  based 
on  the  theory  of  averages.  An  arithmetical  mean  is  the 
sum  of  all  the  numbers  forming  the  series  of  figures  under 
consideration  divided  by  their  number  without  reference 
to  their  weight  or  relative  importance  among  themselves. 
This  method  has  been  applied  to  the  results  of  sampling, 
with  most  unhappy  consequences.  Thus,  let  the  following 
series  of  figures  represent  the  data  obtained  from  sampling 
a  length  of  lOO  ft.  of  gold-bearing  ore  at  intervals  of 
ID  ft. : 

—34— 


SAMPLING    AND    ESTIMITION    OF    ORE    IN    A    MINE 

Width.  Assay. 

4.4  feet  2.35  oz.  per  ton  / 
6.2    "                                             .45  oz.     "       " 

7.6  "  .62  oz.  "  " 

4.0  "  .85  oz.  "  " 

2.5  "  1.02  oz.  "  " 

3.6  "  2.40  oz.  "  " 
2.5  "  4.25  oz. 

.8    "  5.20  oz.    "      " 

1.2      "  4.65   oz.      "         " 

2.2      "  3-21    oz.      "         " 

35.0  25.00 

The  arithmetic  mean  yields  3^  ft.  of  ore  averaging  2^ 
oz.  of  gold  per  ton.  This  is  woefully  wrong.  It  disre- 
gards the  fact,  for  instance,  that  the  third  sample  yielded 
73^  ft.  of  ore  containing  only  0.6  oz.  of  gold,  while  the 
richest  ore,  at  the  eighth  sample,  was  less  than  i  ft.  wide. 

The  geometrical  mean  is  the  sum  of  such  figures  di- 
vided by  their  number,  with  due  allowance  made  for  their 
weight.  This  is  done  in  practice  by  what  many  call  the 
"foot-ounce"  method,  which,  applied  to  'the  foregoing 
example,  works  out  thus : 


Width. 

Assay. 

Foot-ounces. 

4.4  feet 

2.35  oz. 

per 

ton 

I0.34 

6.2    " 

.45  oz. 

« 

2.h 

7.6    " 

.62  oz. 

" 

4.71 

4-^ ;; 

.85  oz. 

" 

3-40 

2.5 " 

1.02  oz. 

" 

2.55 

3.6 " 

2.40  oz. 

" 

8.64 

2.5 " 

4.25  oz. 

« 

10.62 

.8    " 

5.20  oz. 

« 

4.16 

1.2      " 

4.65  oz. 

** 

5-58 

2.2      " 

3.21  oz. 

u 

« 

7.06 

35-0  25.00  59.85 

The  assay  per  foot  of  width  is  1.71  oz.  per  ton,  instead 
of  23/2  oz.  In  matters  of  this  kind  mathematical  reasoning 
confirms  the  conclusions  of  rough  common  sense.  The 
method  just  described  is  based  upon  the  higher  mathe- 
matics, as  the  following  integrations,  kindly  suggested  by 
Mr.  Ross  Hoffman,  will  demonstrate  : 

Problem  i. — To  determine  the  average  value  of  the  sec- 
tion of  vein  A  between  the  samples  whose  widths  are  w^ 
and  W2,  and  values  v,  and  V2,  respectively,  under  the  as- 
sumption that  the  values  vary  gradually  over  the  area 


SAMPLING    AND    ESTIMATION    OF   ORE    IN    A   MINE 

from  Vj  to  v,  in  the  direction  d,  then  at  any  distance  x  from 
Wj  the  value 


also 


v=-(v2-vi)  +  v,     (1) 
d 

d  —  X  x(w2  — Wj) 

ory  = ^ hwi     (2) 


Wi— W2       y— W2  d 

A  =  (Wi  +  W2)  d 


(3) 


x  =  d 


x  =  o 


r  V  y  dx  -^  A  =  average  value  for  the  section  A. 


From  1  and  2  v  y  =  —  (wj  Vi  -f  Wg  v,)  + 
d^ 


^X  X2.  ^  ZA. 

( )(W2  Vj  +  Wt  V2)  +     I Wj  Vj 


d        d 

2  .d 


2X^ 

d 

Wi    Vj  +  W2  Vj 


.-. j  vydx=H \- 

(Wi+W2)do*^  W1+W2 

W2  Vi  +  Wi  V2       Wr  Vi  +  W3  V2             (Wi— W2)    (vj— Vi) 
K = +  K 

W1+W2  Wi4-W2  W1  +  W2 

The  last  term  =  O  when  w^  =  Wj  or  v^  =  v^ 

Under  normal  conditions,  where  ordinary  sampling 
can  be  depended  on  to  give  fairly  approximate  averages, 
this  last  term  will  be  small  enough  to  nes:"lect.  (See  by 
substitutions.)    Hence 

Wi    Vi    +   W2  V2 

can  be  taken  as  the  average  value 

W1  +  W2 

for  section  A. 


-36- 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 


Problem  2. — To  find  the  average  value  from  a  number 
of  samples  taken  as  above,  where  w  equals   width  of 

t^2 


(0| 


sample,  v  equals  value,  d  equals  distance  between  samples 
and  A  the  area  of  various  sections  of  vein  between 
samples : 

^    do  Wo  Vo   +  Wi  Vi- 

Ao  =  (wo  +  Wj  j  —  average  value  for  Aq  = • 

^2  Wo  +  Wi 


.    di                                                    Wi  Vi  +  WaV 
Ai  =  (Wi+W2) "  "      «  Ai  = 


A2  =  (W2   +  W3   j 
A3  =  (W3   +W4) 


d. 


Wj  +  W2 
W2  V2   +  W3  Vj 


(C  u         *•    A 

2  W2  +  W3 

d 


1    «     «  « A  _!:!Z!±Zi:i 


W3+W4 


(See   Prob.  i.) 


2;(AXaverag^valueforA)  _  ^^^^^^^  ^^j^^  ^^^^  ^^ 

(WoVo  +  WiVj-y-  + 

whole  area  sampled  =  - 


Wo  (do) 


+ 


dx 


d. 


(Wi  Vi  +  W,  V2  )  ^-  +  ( W2  V2  +  W3  V3  )  -^  +  (W3  V3  +  W4  V4)  — 

Wi(do  +  di)      W2(di  +  d2)      W3(d2  +  d3)      w^  (d3) 
2  '  2  '  2  '         2 

(wo(do))  (  w,(doH-d,)  ^  j  W2  (di  +  d2)  > 

Wi(do  +  di)  }  (  W2  (di  4-  d2)  ) 


+  v 


(Wo  (do))  (  Wi(do  +  dJ  )  i 

(  W3(d,  +  d3)  ^  j  W4  (da)  1 


(W3(d2  +  d3)  )  (w,  (dg)) 


-37— 


SAMPLING    AND    ESTIMATION    OF   ORE   IN    A    MINE 

The  bracketed  terms  represent  areas  and  may  be  con- 
sidered the  importance  or  weight  factors  with  which  the 
various  sample  values  taken  separately  enter  into  the-  gen- 
eral average  value  for  the  whole  area.  It  is  equivalent  to 
giving  each  sample  value  an  importance  (or  weight)  pro- 
portional to  its  sample  width  multiplied  by  half  the  sum 
of  the  distances  to  the  two  adjacent  samples. 

Averaging  in  this  manner  assumes,  as  in  problem  i, 
that  the  values  between  various  adjacent  samples  change 
gradually. 

If  the  samples  are  taken  equidistantly,  the  above  aver- 
age becomes 

/Wo  N  /W4  X 

Vo  (^— j_|_V,  w,  4-  V2  W2  +  V3  W3  -h  V4  ( — J 

Wo              w^ 
+  Wi  +  W2  -f  W3  

2  2 

which,  with  the  exception  of  the  two  end  samples,  is 
simply  giving  each  value  an  importance  in  the  general 
average  proportional  to  the  width  of  its  sample.  The  two 
end-sample  values  are  shown  to  have  half  this  importance, 
though  in  practice,  with  many  samples  under  considera- 
tion, it  is  customary  to  give  the  two  end  samples  the  full 
importance  proportional  to  their  respective  widths. 

In  the  calculations  for  tonnage  the  cubic  feet  of  ore 
are  converted  into  tons  on  the  basis  of  a  certain  specific 
gravity;  thus,  quartz  is  usually  taken  at  15  cub.  ft.  per 
ton,  while  ores  containing  sulphides  are  rated  at  8  to  12 
cub.  ft.  It  is  not  unusual  to  guess  at  this  proportion,  be- 
cause experience  does  enable  an  engineer  to  approximate 
the  correct  figure  f-»irly  well.  But  it  is  a  dangerous  prac- 
tice. Approximations  should  never  suffice  where  greater 
accuracy  is  possible.  It  is  well  to  weigh  a  series  of  meas- 
ured pieces  of  ore  or  to  determine  actually  the  specific 
gravity  of  a  few  pieces  of  average  vein-stuff.  Surface 
quartz,  on  account  of  its  cellular  structure,  may  require 
20  cub.  ft.  to  weigh  a  ton.  Pyritic  ores  will  vary  to  an 
extent  hardly  appreciable  even  to  the  experienced  eye. 
Grievous  errors  in  tonnage  estimates  have  been  caused 
by  the  assumption  of  an  incorrect  basis  of  calculation. 

-38- 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

A  minor  source  of  error  is  sometimes  created  by  the 
occurrence  in  the  vein  of  numerous  cavities,  or  Vugs/ 
as  the  miners  call  them.  In  some  cases  they  form  a  very 
appreciable  proportion  of  the  space  occupied  by  the  lode, 
so  that  they  are  apt  to  lessen  the  tonnage  obtainable  from 
a  block  of  stoping  ground. 


THE  QUESTION  OF  "HIGH  ASSAYS.'' 

In  these  calculations,  whether  they  be  mathematical  or 
rough-and-ready,  it  is  assumed  that  where  two  adjoining 
samples  vary  there  is  a  gradation  from  one  to  the  other. 
Generally  the  assumption  is  warrantable,  but  occasionally 
the  difference  between  any  two  such  adjoining  samples  is 
so  marked  as  to  require  further  consideration.  In  the 
sampling  of  gold  mines,  especially  where  the  metal  occurs 
in  a  native  condition  and  visible  to  the  eye,  there  will  be  a 
few  very  high  results  which  affect  the  average  of  a  run  of 
samples  to  an  extent  quite  out  of  proportion  to  the  im- 
portance of  a  single  sample. 

This  question  of  "high  assays"  demands  careful  dis- 
cussion. It  can  be  viewed  from  two  standpoints :  the  seller 
of  a  mine  is  apt  to  argue  that  if  you  are  going  to  base  an 
estimate  of  the  ore-reserves  upon  the  results  of  sampling 
and  assay  you  ought  to  take  all  the  data  without  favor,  just 
as  they  come,  and  that  to  eliminate  high  assays  is  no  more 
fair  than  it  would  be  to  omit  poor  ones ;  to  this  the  engi- 
neer, representing  the  interests  of  a  possible  purchaser, 
will  reply  that  the  occasional  very  high  assays  are  largely 
accidental,  that  they  affect  the  estimates  to  a  degree  out  of 
proportion  to  their  weight,  and  to  an  extent  by  no  means 
comparable  to  the  omission  of  an  equal  number  of  poor 
results;  finally,  he  will  murmur  under  his  breath  some- 
thing about  a  factor  of  safety  being  necessary. 

In  the  first  place,  it  is  incumbent  upon  the  engineer  to 
find  out  whether  these  occasional  high  results  do,  or  do 
not,  represent  the  average  of  the  ore,  not  at  the  particular 
place  sampled  only,  but  over  the  whole  space  or  interval 
which  that  sample  is  supposed  to  represent.    Each  sample 

—39— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

in  the  final  calculation  speaks  not  for  a  spot,  but  for  a 
section  of  the  vein,  the  length  of  which  is  the  interval  be- 
tween samples  and  the  height  is  the  distance  separating  it 
from  the  next  working  overhead  or  underfoot ;  thus,  when 
the  distance  between  samples  is  lo  ft.  and  the  next  series 
of  samples  is  on  a  level  lOO  ft.  distant,  above  or  below, 
then  the  section  represented  by  a  single  sample  contains 
10  by  100  ft.,  or  i,ooo  sq  ft.,  which,  if  the  vein  be,  for 
example,  3  ft.  wide,  may  contain  over  200  tons  of  ore. 
In  practice  the  responsibility  of  one  series  of  such  samples 
is  shared  by  that  of  a  corresponding  series  taken  on  the 
level  above  or  below;  nevertheless,  it  is  obvious  that  a 
single  sample  represents  something  very  different  to  the 
mere  spot  or  place  in  a  particular  working.  In  order  to 
answer  the  question  whether  an  individual  high  assay 
is  accidental  or  representative,  the  only  thing  to  do  is  to  re- 
sample  at  the  same  place ;  if  the  result  is  confirmatory,  then 
evidently  that  spot  is  as  rich  as  the  first  sampling  indicated. 
But  this  does  not  determine  whether  the  high  values  gra- 
date on  either  side  toward  the  adjoining  samples.  Thus,  if 
A,  B  and  C  in  the  diagram  represent  three  points,  10  ft. 
apart,  which  have  been  sampled  with  the  results  shown, 
then,  if  B  assayed  10.40  oz.  and  was  re-sampled  with  8.22 
oz.   as  the  assay   return,   the  next  question   arising  is 


A  «* 10  ft. ►  B  * 10  f fer- 


^ 1^ 

1.65  oz,  10.40  oz.  0.86  oz. 

8.22  oz. 


whether  the  ore  midway  between  B  and  A,  or  B  and  C,  is 
correspondingly  intermediate  in  value ;  for  this  is  what  the 
calculations  assume.  For  this  reason  it  is  best  to  check 
the  high  assays  by  taking  further  samples  at  these  inter- 
mediate places,  thereby  finally  settling  the  query  whether 
the  high  results  are  trustworthy  factors  in  estimating  the 
average  value  of  the  ore-bodies. 

The  treatment  of  "high  assays"  has  caused  much  dis- 
cussion among  engineers,  especially  when  they  are  en- 
countered in  the  course  of  a  sampling,  the  results  of  which 

—40— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

are  to  decide  the  price  of  a  mine.  The  seller,  or  his  repre- 
sentative, is  apt  to  suggest  that  a  re-sample  of  poor  spots 
is  also  in  keeping  with  the  theory  of  the  whole  business, 
and  to  this  I  would  say  that  where  a  barren  or  nearly 
barren  result  is  obtained  amid  a  series  of  samples  indi- 
cating good  ore  it  is  correct  to  re-sample  such  a  place  or 
places.  The  occasional  barren  and  the  occasional  high 
assay  return,  however,  are  not  comparable  in  their  effect 
upon  the  estimates.  In  the  first  place,  the  "nil"  or  "trace" 
indicates  the  entire  absence  of  the  metal  and  has  not  the 
accidental  feature  arising  from  the  presence  of  a  stray 
speck  of  gold,  which  may  cause  the  abnormally  high  assay ; 
further,  one  very  high  assay  affects  the  calculations  more 
than  a  number  of  poor  ones,  supposing  both  to  be  devia- 
tions from  fact,  as  can  be  easily  shown,  thus :  Take  the 
following  series  of  samples  and  assay  returns : 


Width. 

Assay. 

Foot-ounces. 

7  feet 

1.22  oz.  gold 

8.54 

4     " 

.54  oz. 

2.16 

12       " 

1.20  OZ. 

« 

14.40 

8     " 

2.20   OZ. 

« 

17.60 

5.3" 

27.40  OZ. 

144.22 

7     " 

1.60  OZ. 

11.20 

3.5" 

.95  OZ. 

332 

6     " 

.39  OZ. 

2.34 

6     " 

trace 

7.2" 

trace 

« 

... 

66.0  3550  203.78 

The  average  of  the  ore,  using  these  data,  is  slightly 
over  3  oz.  per  ton.  If,  however,  the  fifth  sample,  which  is 
very  high,  is  eUminated,  the  average  drops  to  less  than  I 
oz.  per  ton,  while,  on  the  other  hand,  if  the  high  sample  is 
retained  and  the  last  two  poor  places  are  omitted,  the  re- 
sult is  affected  to  a  much  less  degree,  the  average  then 
being  about  3^  oz.  per  ton. 

In  re-sampling  these  apparently  very  rich  spots,  it 
will  be  found  that  three  contingencies  may  occur;  the 
result  may  be  corroborated  or  it  may  be  proved  to  be 
accidental,  while,  should  every  high  assay  be  found  in- 
correct, then  there  is  a  third  possibility  to  be  considered, 
namely,  whether  the  samples  have  been  tampered  with. 

—41— 


SAMPLING    AND    ESTIMATION    OF    ORE   IN    A   MINE 

If  out  of  a  considerable  number  of  high  results  not  one  is 
confirmed,  then  it  is  time  to  look  around.  If  some  are 
confirmed  by  re-sampling,  while  others  are  not,  then, 
obviously,  the  values  in  the  ore  are  erratic,  and  a  correc- 
tion must  be  made  for  each  case ;  if  most  of  the  high  re- 
sults are  approximately  repeated  by  the  second  sam- 
ple, then  it  is  evident  that  the  first  sampling  was  correct, 
aoid  that  the  ore  is  very  rich  in  spots ;  intermediate  sam- 
ples must  be  taken,  and  when  this  is  done  the  data  for 
calculations  will  be  complete.  It  is  wise  to  take  intermedi- 
ate samples,  both  when  the  high  assays  are  wholly,  and 
when  they  are  only  partially,  confirmed. 

It  has  been  argued  by  certain  engineers  that  the  mill 
or  smelter  always  fails  to  confirm  the  very  high  assays 
occasionally  obtained  in  sampling  and  that,  therefore, 
they  should  be  omitted  as  a  factor  of  safety.  This  is  not 
a  fact.  Exceptions  occur.  In  the  case  of  the  Argentine 
lode  of  the  Tomboy  Mining  Company,  at  Telluride,  Colo., 
a  careful  sampling  of  a  block  of  ground,  at  intervals  of 
lo  ft.,  yielded  an  average  of  $7  per  ton,  all  rich  spots 
carrying  visible  gold  being  avoided.  This  work  was 
done  by  an  inspecting  engineer  of  recognized  capacity; 
nevertheless,  the  actual  mill  returns  were  $28  per  ton. 
Nor  is  this  inexplicable  when  the  nature  of  the  ore  is 
considered.  The  writer  saw  a  piece  weighing  25  lb., 
yellow  with  finely  disseminated  gold,  so  that  it  contained 
$100  worth  of  the  native  metal,  which  came  from  the 
breast  of  a  level  on  a  certain  day — ^and  the  very  next 
day  the  foreman's  sample  from  the  face  of  the  same  drift, 
which  had  been  advanced  two  or  three  feet  during  the  in- 
terval, yielded  only  two-hundredths  of  an  ounce  of  gold 
per  ton.  The  Pandora  vein,  in  the  neighboring  Smuggler- 
Union  mine,  affords  another  exceptional  case;  the  mill 
results  are  usually  higher  than  the  assays,  and  this  is 
explained  as  being  due  partly  to  the  greater  hardness  of 
the  rich  ore,  but  chiefly  because  the  gold  occurs  almost 
entirely  in  coarse  particles  which,  being  readily  visible, 
are  apt  to  be  carefully  shunned  by  the  conscientious  sam- 
pler.    These  instances  are  corroborated  by  the  thor- 

—42— 


SAMPLING   AND    ESTIMATION    OF    ORE    IN    A    MINE 

oughly  experienced  managers  of  the  properties  quoted. 
It  remains  to  add  that  while  it  is  true  of  the  veins  specific- 
ally mentioned,  that  the  assays  are  usually  lower  than 
the  mill  returns,  such  is  not  the  case  with  the  Tomboy 
lode,  which  is  near  the  Argentine,  nor  is  it  true  of  the 
Smuggler  lode,  which  is  intersected  by  the  Pandora  in 
the  Smuggler-Union  mine.  The  above  are,  however, 
well-authenticated  exceptions,  which  serve  effectually  to 
undermine  a  generalization  which  is  the  last  resort  of  a 
timid  engineer.  Not  that  the  factor  of  safety  is  to  be 
discarded,  quite  the  contrary;  in  all  engineering  such  a 
precautionary  measure  is  imperative,  but  do  not  intro- 
duce it  disguised ;  face  the  facts,  state  them,  and  then  in- 
troduce your  factor  of  safety  without  any  circumlocu- 
tion. 

Why  do  sampling  results  differ  from  the  mill  returns? 
In  the  unusual  instances,  just  mentioned,  the  difference 
was  traceable  to  conditions  which  are  relatively  rare,  but 
are  always  within  the  range  of  possibility.  There  is  no 
doubt  in  my  mind  that  the  usual  experience  of  finding 
the  mill-extraction  below  the  estimates,  based  upon  sam- 
pling and  assay,  is  due  to  the  fact  that  rich  ore  is 
ordinarily  in  the  softer,  more  crumbly  parts  of  a  lode,  ov 
it  is  associated  with  sulphides  which  are  not  only  easier 
to  break  than  quartz,  but  they  also  make  a  shining  mark 
which  invites  the  blow  of  the  moil  and  hammer.  This 
aspect  of  the  inquiry  emphasizes  the  weak  factor  in  sam- 
pling, it  is  not  absolutely  mechanical,  a  man  and  not  a 
machine  does  the  work,  and  the  results  partake  of  that 
liability  to  error  which  is  essentially  human. 

It  may  be  asked,  in  conclusion,  what  course  should 
be  adopted  with  high  assays  when  it  is  not  possible  to  re- 
sample?  Frequently,  the  samples  are  not  assayed  near 
the  mine,  but  are  taken,  or  sent,  by  the  engineer  to  a  re- 
liable assayer  living  in  a  distant  locality.  Time,  an  ele- 
ment in  all  business  matters,  may  prevent  the  engineer 
from  returning  to  the  mine  to  take  further  samples. 
This  is  a  dilemma  not  infrequent  in  current  practice. 
Judgment  and   experience  must   decide.     The   relative 

—43— 


SAMPLING   AND    ESTIMATION    OF    ORE    IN    A    MINE 

frequency  of  high  assays,  the  degree  to  which  they  affect 
the  final  estimates,  the  character  of  the  ore,  the  resuks 
of  sampHng  as  compared  to  the  actual  recorded  average 
of  the  mine,  the  character  of  the  work  done  by  the  engi- 
neer and  his  assistants;  these  and  similar  factors  will 
determine  the  decision.  One  cannot  lay  down  rules 
to  direct  any  man's  judgment. 

It  may  be  permitted  to  say  that  if  you  are  uncertain 
of  your  results,  don't  use  them.  Do  not  let  yourself  be 
hurried,  either  by  your  client  or,  as  is  more  probable,  by 
the  vendor,  to  committing  yourself  to  a  decision  based 
upon  data  the  accuracy  of  which,  in  your  own  mind,  is 
in  any  doubt.    It  is  better  to  he  sure  than  sorry. 


THE    POSSIBLE    DISCREPANCIES    BETWEEN    SAMPLING    AND 
MININa 

In  sampling  the  workings  it  is  necessary  that  the  en- 
gineer should  have  an  eye  to  the  manner  in  which  the 
mine  is  being  worked.  It  is  useless,  for  instance,  to  sam- 
ple two  feet  of  vein-matter  without  any  regard  to  the 
fact  that  in  stoping  it  is  the  practice  to  remove  a  width  of 
four  feet.  Unless  judgment  is  exercised,  the  samples 
are  apt  to  represent  cleaner  ore  than  the  output  of  the 
mine,  so  that  the  estimates  based  upon  them  will  be  mis- 
leading. The  material  sent  for  treatment  to  mill  or  smel- 
ter may  differ  from  the  ore  composing  the  lode  as  seen 
in  the  mine,  in  several  respects.    Thus : 

A.  The  lode  may  be  smaller  than  the  mmimum  width 
removed  in  stoping,  so  as  to  necessitate  the  mining 
of  a  certain  portion  of  barren  rock. 

B.  The  lode  may  be  as  wide  as  tlfe  convenient  size 
for  stoping,  but  it  may  have  a  casing  of  soft  rock, 
w^hich  breaks  down  with  the  ore  and  becomes  min- 
gled with  it  so  as  to  increase  the  tonnage  and  di- 
minish the  average  yield  per  ton. 

C.  The  lode  may  be  built  of  one  or  more  streaks  of 
rich    ore    irregularly    distributed    through    it,    so 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

that  in  stoping  it  is  necessary  to  break  down  a  large 
and  variable  width. 

In  the  preliminary  trip  through  the  mine  the  engineer 
will  be  able  to  find  out  by  observation,  together  with  the 
information  obtainable  from  either  the  foreman  or  the 
manager,  how  the  ground  is  being  worked,  and  how  the 
ore  is  being  handled.  If  he  does  not  secure  these  data  at 
the  preliminary  examination  he  will  light  upon  them  in 
the  course  of  his  investigations  into  costs,  for  without 
them  the  relative  expenditures  at  mine  and  mill  will  be 
contradictory,  if  not  unintelligible. 

Concerning  the  conditions  first  mentioned,  under  the 
heading  A,  it  is  obvious  that  the  width  broken  in  stoping 
has  a  minimum,  which  in  ordinary  practice  varies  accord- 
ing to  the  method  employed;  that  is,  with  hand  labor  a 
width  of  two  to  three  feet  will  ordinarily  suffice;  while 
at  least  three  to  four  feet  is  necessary  when  machine 
drills  are  used.^  In  both  cases  this  minimum  width  will 
be  exceeded  if  the  rock  is  very  hard  or  if  it  breaks  along 
easy  lines  of  fracture,  which  cause  more  rock  to  be  taken 
down  in  blasting  than  is  absolutely  required  for  mere 
convenience  in  working.  If  the  streak  of  ore  is  less  than 
the  width  of  the  stope  there  will  be,  inevitably,  an  ad- 
mixture of  waste,  except  in  the  rare  cases  where  it  is 
practicable  to  strip  the  pay-streaks,  that  is,  stope  the 
country  without  removing  the  vein,  which  is  subsequent- 
ly taken  down  separately.  This  method,  which  is  cov- 
ered by  the  Cornish  term  "resue,"  is  excellent  when  the 
vein  of  ore  is  narrow,  but  it  depends  for  success  upon 
the  ore  being  closely  attached  or  ''frozen"  to  one  of  the 
vein  walls,  so  that  it  will  not  be  shaken  down  when  the  ad- 
joining waste  is  shot  away.  In  many  mines  in  Colorado, 
Montana  and  Idaho,  especially  in  the  case  of  narrow 
streaks  of  very  rich  silver  ore,  it  is  not  unusual  to  spread 
canvas  or  sacking  along  the  floor  of  the  stope  and  then 
break  dawn  the  ore  upon  it,  which  in  this  way  is  kept 
free  from  admixture  with  waste,  and  can  be  sacked  with- 


'A  small  (254 -in.  cylinder)   drill  usually  requires  a  stope  of  3^4  to  4  ft. 
in  width,  the  large  air  drills  (with  zyi-'m.  cylinder)  take  from  4J4  to  5  ft. 

—45— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

out  any  sorting.  The  canvas  is  covered  for  protection 
with  6  by  3  by  i2-in.  timbers.  In  such  a  mine  as  this  the 
sampling  of  the  vein  itself  would  give  data  which  closely 
approximate  the  actual  returns  from  ore  sent  away  to  the 
smelter. 

When  it  is  not  possible  to  mine  the  ore  separately  from 
the  waste  it  is  usual  to  pass  it  over  the  sorting  tables, 
where  the  waste  is  picked  out  by  hand.  In  the  deter- 
mination of  the  percentage  thus  eliminated  lies  the  diffi- 
culty of  getting  the  proper  relation  between  the  assays  of 
the  ore  as  sampled  and  the  value  of  the  material  as  sent 
to  the  reduction  works.  Unless  this  is  determined,  and 
the  sampling  is  corrected  in  accordance,  the  figures  of 
the  engineer  will  not  harmonize  with  the  results  of  future 
operations.  The  most  practical  way  to  find  out  the  per- 
centage sorted  out  is  to  measure  the  ground  stoped  dur- 
ing a  given  period,  and,  knowing  the  tonnage  extracted 
from  such  a  particular  block  of  ground,  to  determine  the 
average  width  of  clean  ore  actually  obtained  from  that 
block.  If  the  average  width  of  the  vein  is  fairly  consist- 
ent, one  can,  from  these  data,  deduce  the  amount  of  waste 
which  gets  mingled  with  the  pay-streak  and  then  cor- 
rect the  assay  returns,  from  sampling,  accordingly. 

However,  gold-veins  are  rarely  uniform  in  width,  and, 
moreover,  any  given  mine  may  contain  several  lodes, 
the  output  of  which  is  not  kept  separate.  In  such  local- 
ities as  Kalgoorlie  and  Cripple  Creek,  where  the  veins 
are  rich,  erratic,  and  so  diffuse  in  th'eir  mineralization  as 
to  -make  it  compulsory  to  break  down  a  great  deal  of 
waste  in  extracting  a  small  width  of  clean  ore;  where, 
also,  the  large  mines  include  within  their  boundaries  two 
or  three  distinct  lodes  of  variable  width  and  richness,  it 
becomes  an  extremely  difficult  task  for  an  engineer  to 
determine,  within  the  short  period  of  his  inspection,  to 
what  degree  the  vein-stufif  is  mixed  up  with  waste.  An 
example  will  emphasize  the  matter  under  discussion. 
Take  a  lode  or  series  of  lodes  having  an  average  width 
of  one  foot  of  four-ounce  ore.  Assume,  as  is  usually  the 
case,  that  a  width  of  at  least  four  feet  of  rock  is  actually 

—46— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

removed  in  mining,  so  that  one  foot  of  ore  is  shot  down 
with  three  feet  of  waste.  If  you  sample  the  vein  alone 
you  get  an  average  of  four  ounces  of  gold  per  ton ;  i{ 
you  sample  the  four  feet  of  stoping  width  you  get  an 
average  of  one  ounce  per  ton,  unless  there  happen  to  be 
one  or  more  stringers  of  ore  outside  the  main  pay- 
streak,  not  rich  enough  to  be  worked  on  their  own  ac- 
count, but  near  enough  to  the  vein  proper  to  be  included 
within  the  ground  which  is  removed.  In  this  case  the 
average  may  become  slightly  higher  than  the  figure 
which  is  based  on  the  supposition  that  the  extra  three 
feet  is  barren  material.  That  is  a  detail.  The  point  to 
be  emphasized  is  that  the  product  of  the  mine  will  be 
more  than  the  tonnage  based  on  one  foot  of  ore  and 
considerably  less  than  the  tonnage  based  upon  the  stoping 
width;  more  is  broken  than  one  foot,  because  mining 
does  not  copy  sampling  methods,  and  less  is  shipped 
than  four  feet,  by  reason  of  the  sorting  which  intervenes 
between  stope  and  smelter.  Similarly,  as  to  the  average 
value,  the  production  of  the  mine  will  average  less  than 
the  four  ounces  in  the  clean  ore,  but  more  than  the 
one  ounce  which  is  the  assay  value  of  the  stoping  width. 

At  Cripple  Creek,  and  in  other  districts  where  the  costs 
of  transport  and  treatment  are  high,  this  four  feet, 
or  more,  of  ore  is  sent  to  the  ore-house  and  is  sorted  by 
hand.  It  may  have  been  first  culled  underground,  the 
large  pieces  of  waste  being  retained  on  the  stulls.  This 
is  the  practice  in  most  mines.  In  any  event,  a  certain  pro- 
portion of  waste  is  thrown  out,  and  it  is  only  the 
remainder  that  is  milled  or  marketed.  What  is  that  pro- 
portion which  is  taken  out? 

Here  lies  the  difficulty  which  has  been  at  the  bottom  of 
many  an  erroneous  estimate.  If  the  engineer  is  pressed 
for  time,  by  reason  of  agreements  made  between  the 
buyer  and  seller  of  the  mine,  he  will  not  have  the  oppor- 
tunity to  conduct  such  tests  as  would  give  him  the 
requisite  data.  The  management  is  apt  to  have  loose 
ideas  on  the  subject;  the  ratio  of  waste  eliminated  to 
clean  ore  actually  shipped  will  vary  from  week  to  week, 

—47— 


SAMPLING   AND    ESTIMATION    OF   ORE    IN    A   MINE 

just  as  the  different  stopes  underground  will  change  from 
time  to  time,  so  that  it  is  rarely  possible  within  the  period 
of  a  brief  examination  to  get,  save  at  second-hand,  at  the 
percentage  which  so  seriously  affects  all  calculations. 
Under  these  conditions  the  only  thing  to  do  is  to  state  the 
results  just  as  they  are  obtained,  explaining  how  they  are 
obtained,  and  then  make  the  correction  which  judgment 
dictates.  If  the  vein  has  been  sampled  by  itself,  then  an 
addition  for  the  unavoidable  admixture  with  waste  must 
be  estimated,  the  amount  being  based  upon  the  observa- 
tion both  above  and  underground.  If  the  sampling  has 
included  the  full  stoping  width,  then  a  similar  correction 
must  be  made  for  the  amount  of  material  subsequently 
removed  by  sorting,  otherwise  the  statement  of  sampling 
returns  may  prove  unintelligible,  it  being  not  uncommon 
for  the  average  value  of  the  ore  as  stoped,  before  sort- 
ing, to  be  less  than  the  total  working  costs. 

However,  except  in  the  unpleasant  circumstance  of 
limited  time  due  to  the  unavoidable  exigencies  of  busi- 
ness, which  unfortunately  do  often  hamper,  hinder  and 
obstruct  the  engineer  in  his  professional  work,  it  is  pos- 
sible to  arrive  at  the  ratio  of  ore  and  waste,  and,  there- 
fore, to  formulate  accurate  estimates.  It  requires  time 
and  it  needs  money,  but  in  the  examination  of  large 
mines  nowadays  it  is  appreciated  by  financiers  and  syndi- 
cates employing  first-class  engineers  that  they  must 
make  provision  for  plenty  of  time  and  money  in  order 
to  get  good  men  to  do  good  work.  This  may  seem  a 
digression,  but  it  vitally  affects  the  consideration  of  this 
branch  of  the  subject  under  discussion. 

When,  therefore,  conditions  do  permit  of  every  precau- 
tion, the  engineer  will  cause  the  ore,  as  it  comes  to  the 
surface,  to  be  weighed,  he  will  get  the  weight  of  the  clean 
ore  after  sorting,  and  he  will  check  this  with  the  weight 
of  the  material  which  goes  over  the  dump.  He  will 
also  take  samples  of  the  dump  to  correct  his  estimate  of 
the  average  values.  The  weighing  and  sampling  will,  of 
course,  be  done  by  his  own  men.  He  will  also  take 
careful  note  of  any  waste  which  is  retained  underground, 

—48— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

as  is  usually  the  case  in  a  well-managed  mine.  If  this  i$ 
done  during  the  space  of  a  month  or  so,  which  is  the 
minimum  period  permissible  for  an  important  examina- 
tion, he  will  have  data  enough  to  enable  him  to  correct 
the  results  secured  from  the  sampling  of  the  mine.  In 
the  suppositious  case  quoted  above,  the  one  foot  of 
four-ounce  ore  would  be  mined  (and  sampled)  with  three 
times  as  much  waste,  one-half  of  the  material  thus  broken 
would  be  sorted  out  at  surface,  and  with  it  would  go  some 
of  the  fines,  so  that  the  net  result  might  be  about  two  feet 
of  ore  assaying  1.75  oz.  and  two  feet  of  waste  assaying 
about  0.25  oz.  per  ton. 

These  final  estimates  should  be  borne  out  by  the  future 
record  of  the  mine.  As  a  safeguard,  however,  the  engi- 
neer ought  to  state  in  detail,  in  his  report,  how  he 
arrived  at  his  figures,  lest,  later  on,  an  ignorant  direc- 
torate or  an  unscrupulous  manager  should  either  work 
more  than  a  proper  width,  in  order  to  get  an  increase  in 
tonnage,  or  mine  only  the  richest  portion  of  the  lode 
with  a  view  to  a  brief  period  of  inflated  returns.  Such 
things  have  happened,  and  honest  men  have  been  drawn 
into  the  blame  which  followed. 

Coming  to  the  consideration  of  the  second  point, 
specified  under  B,  it  is  evident  that  here  also  there  is 
room  for  error.  In  most  lodes  of  gold-bearing  quartz, 
where  sorting  is  out  of  the  question  on  account  of 
the  low  grade  of  the  ore  and  the  uncertain  dissemina- 
tion of  the  gold,  it  is  not  possible  to  break  the  ore  in 
the  stopes  as  clean  as  it  is  broken  in  the  course  of 
sampling.  Should  the  lode  be  free  from  an  admixture 
of  country  it  is,  nevertheless,  rarely  practicable  to  stope 
It  without  bringing  down  some  of  the  encasing  rock. 
This  is  slight  in  some  instances  of  very  well-defined 
quartz  veins,  large  enough  to  admit  of  a  full-sized  stope 
and  so  separated  by  selvages  from  the  wall-rock  as  to 
come  clear  away.  In  most  instances,  however,  the  stop- 
ing  will  include  a  large  proportion  of  waste,  and  when  it 
happens,  as  is  not  infrequent,  especially  with  big,  generous 
ore-deposits,  that  the  lode  throws  out  branch-veins  or  is 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

enriched  by  feeders,  then  it  will  be  found  that  there  is  a 
very  considerable  admixture  of  comparatively  barren  rock. 

Circumstances  such  as  these,  overlooked  or  under- 
estimated, have  been  at  the  root  of  the  differences  be- 
tween the  estimates  of  capable  engineers  and  the  subse- 
quent record  of  the  mines  they  have  reported  upon. 
Managers,  as  a  rule,  like  to  emphasize  the  clean-cut  char- 
acter of  their  operations,  and  are  apt  to  accentuate  the 
fact  that  the  ore  "breaks  easy"  and  free  from  wall-rock. 
Moreover,  in  all  mining,  there  is  an  element  of  the  unex- 
pected, which  in  this  case  takes  the  form  of  the  breaking 
away  of  "slabs"  of  wall-rock,  the  admixture  of  large 
pieces  of  "casing,"  the  occasional  "horse"  or  intru- 
sion of  barren  rock  amid  a  width  of  ore,  and  other 
contingencies,  all  of  which  tend  inevitably  to  an  in- 
crease of  tonnage  and  a  diminution  in  the  average 
value  of  the  output.  One  must  look  for  these  features 
in  a  mine  and  obtain  data  sufficient  to  warrant  an  esti- 
mate of  their  perturbing  influence  upon  the  accuracy  of 
those  calculations  which  are  at  first  based  solely  on 
the  results  of  sampling. 

The  third  subdivision,  C,  includes  a  large  proportion 
of  the  big  low-grade  lodes  from  which  so  much  of  the 
metallic  wealth  of  the  world  is  derived.  An  examina- 
tion of  a  mine  is  usually  confined  to  those  workings 
which  are  confessedly  profitable  or  likely  to  become  so. 
The  non-payable  workings  are  neglected.  In  actual  min- 
ing, however,  a  level  or  a  raise  is  apt  to  be  carried 
forward  through  poor  ground  in  the  hope  of  encounter- 
ing better  ore,  and  it  is  not  uncommon  to  remove  blocks 
of  unprofitable  ground  lying  between  good  stopes  be- 
cause of  practical  convenience  in  working.  In  this  way 
there  is  a  tendency  to  vitiate  the  results  of  sampling,  if 
not  corrected  in  accordance  with  a  recognition  of  such 
indubitable  facts.  Moreover,  in  the  working  of  large  de- 
posits of  low-grade  ore,  where  the  width  to  be  removed 
is  not  determined  by  well-defined  boundaries,  but  is  left 
ito  the  arbitrament  of  the  assay,  as  a  consequence  of  a 
diffused  impregnation  of  ore,  there  is  a  tendency  to  in- 

—50— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

crease  the  stoping  width.  The  result  of  a  steady  diminution 
in  working  costs,  due  to  better  management,  improved 
equipment  and  more  favorable  economic  conditions,  is  to 
permit  of  the  exploitation  of  poorer  ores.  This  leads  to 
the  utilization  of  neglected  stopes,  previously  considered 
improfitable,  and  the  enlargement  of  the  stoping  width  so 
as  to  include  more  and  more  of  the  outer  poorer  edges  of 
the  lode.  An  increase  in  the  capacity  of  the  mill  is  usually 
followed  by  a  drop  in  the  average  value  of  the  output. 
On  the  Rand  this  tendency  has  become  very  marked ; 
thus,  for  exam-pie,  most  of  the  original  estimates  of  the 
life  of  particular  mines  were  based  on  ore-reserves  cal- 
culated upon  the  basis  of  a  certain  width  of  *  banket,' 
but  since  the  substitution  of  rock-drills  for  hand  labor 
it  has  been  found  advisable,  consequent  upon  observa- 
tion and  experience,  to  increase  the  stoping  width  by  as 
much  as  50  per  cent.  This  increase  has  necessitated  the 
breaking  of  a  larger  amount  of  barren  rock,  but  a  com- 
pensating factor  has  been  found  in  the  introduction  of 
revolving  circular  tables,  which  facilitate  the  picking  out 
of  the  waste  by  natives. 

These  are  considerations  which  the  engineer  must 
keep  in  mind.  He  is  not  expected  to  write  next  year's 
almanac,  but  if  he  is  to  be  justified  by  the  actual  record 
of  the  mine,  his  report  must  be  aided  by  some  of  that 
farsightedness  and  shrewdness  which  is  necessary  to  a 
successful  meteorological  forecast. 

ESTIMATION    OF   ORE-RESERVES. 

It  has  been  well  said  that  **if  you  want  to  arrive  at  in- 
telligible issues — not  to  say  conclusions — in  any  discus- 
sion, begin  by  settHng  the  meaning  of  the  terms  you  are 
going  to  use."^  This  is  particularly  necessary  in  dis- 
cussing a  subject  which  suffers  from  the  want  of  defini- 
tion. "Ore  in  sight"  has  become  one  of  those  nebulous 
phrases  which  are  only  noise  and  smoke.  It  is  impera- 
tive that  we  have  a  clear  conception  of  the  fact  signified 


^Dr.  Thompson,  late  master  of  Trinity  College,  Cambridge. 
—SI  — 


SAMPLING   AND    ESTIMATION    OF    ORE   IN    A    MINE 

by  the  technical  terms  we  employ;  when  we  find  that 
they  are  robbed  of  sense  as  soon  as  we  dissect  them,  it 
devolves  upon  us  to  discard  them  and  seek  for  better 
ones.  "Ore  in  sight,"  as  used  to  describe  the  ore- 
reserves  of  a  mine,  is,  if  taken  literally,  a  contraction  in 
terms,  and,  if  taken  otherwise,  it  has  an  elasticity  which 
has  caused  many  to  stretch  it  until  it  has  become  so 
indefinite  as  to  include  the  ore  which  is  beyond  even  the 
most  imaginative  vision. 

In  order  to  express  the  results  of  careful  sampling 
and  estimation  by  a  phrase  which  will  at  once  convey 
its  meaning,  even  to  the  untechnical,  I  would  suggest 
"ore  in  reserve  and  ready  to  be  broken,"  or  "ore  ready 
for  stoping."^  This  would  cover  those  parts  of  a  mine 
which  have  been  so.cut  up  by  systematic  workings  as  to 
permit  of  very  close  calculations.  Next  would  come  the 
blocks  of  ground  incompletely  developed,  but  known 
to  carry  ore,  which,  in  character  and  persistence,  are 
similar  to  the  main  portion  of  the  mine.  These  could  be 
covered  by  the  term  "probable  ore-reserves."  Beyond 
this  point  a  careful  engineer  will  not  go,  avoiding  "pos- 
sible ore-reserves,"  or  any  similar  phrase,  as  a  snare  of 
the  devil,  because  of  the  great  likelihood  that  his  client, 
or  the  shareholders  who  may  follow,  will  disregard  the 
qualifying  adjective  and  commit  him  to  a  meaning" 
which  he  did  not  intend  to  convey.  When  it  comes  to 
the  chances  of  development,  itself  a  most  important  part 
of  the  engineer's  exercise  of  judgment,  it  will  be  best 
to  include  the  consideration  of  this  aspect  of  the  inquiry 
under  the  paragraphs  which  deal  with  "the  future  pros- 
pects  of  the  mine." 

That  is  reserved  which  is  stored  for  future  use,  there- 
fore that  which  is  to  be  used  forthwith  is  not  in  reserve, 
from  which  it  follows  that  by  "ore-reserves"  is  meant 
such  bodies  of  ore  as  are  kept  back  for.  the  present  with 


^This  discussion  of  terms  was  written  some  time  ago,  before  Mr.  Argall 
brought  forward  the  sensible  suggestions  which  appeared  in  The  Engineer- 
ing AND  Mining  Journal  of  February  14,  1903.  They  will  be  found  on 
page  76  of  this  book. 

— 52 — 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

a  view  to  future  use,  when  those  are  exhausted  which 
are  now  being  stoped.  A  mine  which  is  being  stoped 
at  a  rate  so  rapid  in  ratio  to  its  development  that  it  has, 
at  any  given  time,  only  enough  ore  to  keep  the  mill 
going  for  a  few  palpitating  moments  cannot  be  said  to 
have  any  "ore-reserves."  The  idea  of  the  latter  includes 
a  reservoir  of  supply  not  to  be  exhausted  at  short  no- 
tice. As  a  consequence  of  these  considerations  it  is 
obvious  that  the  amount  of  ore  which  warrants  being 
entitled  "reserves"  will  depend  upon  the  relative  size 
of  the  reduction  plant  which  is  attached  to  the  mine. 

At  the  outset  it  is  permissible  to  quote  the  famous 
dictum  of  President  Cleveland,  in  referring  to  the  tariflf: 
"It  is  a  condition  which  confronts  us,  not  a  theory."^ 
Any  attempt  to  establish  uniformity  of  procedure  in  the 
estimation  of  ore-reserves  is  bound  to  break  down  be- 
cause it  disregards  the  inevitable  diversity  of  the  condi- 
tions which  obtain  in  mining.  Mines  differ,  as  men  do. 
A  safe  presumption  in  one  case  is  a  hazardous  guess  in 
another.  During  a  recent  discussion  of  this  subject  it 
was  suggested  that  it  was  desirable  for  mining  engi- 
neers to  agree  upon  certain  general  rules  as  to  the  al- 
lowance to  be  made  in  estimating  ore.  You  might  as 
well  sign  an  agreement  upon  the  percentage  of  trust  to 
be  placed  in  human  nature.  There  are  mines,  the  ore- 
bodies  of  which  are  of  such  a  character  that  it  is  safe 
to  predict  their  persistence  for  several  hundred  feet; 
there  are  others  where  the  lode  is  so  erratic  that  ten  feet 
is  a  dangerous  assumption. 

Examples  will  serve  to  emphasize  the  range  of  varia- 
tion. On  the  Rand  a  bedded  vein  of  gold-bearing  con- 
glomerate extends  for  a  great  distance,  and  over  portions 
of  its  known  length  it  maintains  an  average  tenor  so 
fairly  constant  that  "the  values  of  unworked  portions 
may  be  closely  calculated  from  the  results  achieved  in 
adjoining  developed  mines. "^  In  any  given  mine — 
to  narrow  the  assumption — it  is  quite  safe  to  calculate 

^Messages  of  the  Presidents,  Richardson,'    Vol.  viii,  p.  590. 
'G.  A.  Denny.     The  Deep-Level  Mines  of  the  Rand,  p.  119. 

—S3— 


SAMPLING    AND    ESTIMATION    OF    ORE    I^N    A    MINE 

the  average  value  of  the  ore  in  a  block  of  ground,  say, 
300  ft.  long  and  150  ft.  high,  that  is,  after  this  block, 
150  by  300  ft.,  has  been  sampled  at  intervals  of,  say,  10 
ft.,  on  every  one  of  the  four  sides  of  the  rectangle.  This 
would  be  the  ideal  "ore  in  reserve,"  a  thoroughly  sam- 
pled block,  not  too  big  to  vitiate  the  assumption  of  a 
certain  uniformity  in  value  per  ton.  In  contrast  to  this 
I  would  quote  such  mines  as,  for  example,  the  Seven- 
Thirty  at  Silver  Plume,  Colorado,  or  the  Yellow  Pine, 
near  Boulder,  in  the  same  State,  both  of  them  mines 
which  have  been  at  some  time  richly  productive,  but 
with  a  distribution  of  value,  in  silver  chiefly,  so  varying, 
so  spotty  and  uncertain,  that  careful  sampling,  at  regu- 
lar intervals  of  even  less  than  5  ft.,  around  the  four 
sides  of  a  block  only  50  ft.  square,  would  afford  the 
basis  for  an  estimate  which  at  its  best  would  be  only  a 
reasonable  guess.  Each  mine  must  be  judged  on  its 
merits,  in  each  instance  the  conditions  vary,  and  while 
there  may  be  a  general  similarity  of  method  in  getting 
at  the  various  facts,  there  never  can  be  any  cast-iron 
uniformity  in  the  nature  of  the  inferences  deducible 
from  the  facts.  Experience,  silvered  with  age,  is  the  pre- 
siding judge,  and  will  decide  whether  this  or  that  piece 
of  evidence  is  relevant  or  not.  Nor  is  there  any  room 
for  pessimism  or  for  optimism;  one  is  as  much  out  of 
place  as  the  other ;  indeed,  it  is  not  too  much  to  say  that 
while  a  sanguine  temperament  will  lead  sometimes  to 
exaggerated  expectations,  it  is  equally  true  that  an  over- 
cautious hesitancy  is  to  be  condemned.  Occasionally, 
an  engineer  in  his  effort  to  avoid  risking  his  own  reputa- 
tion is  apt  to  lean  toward  a  timid  conservatism  which 
sacrifices  the  interest  of  his  client.  He  is  engaged  to 
determine  the  facts,  in  the  first  place,  and  then  to  apply 
his  best  judgment  to  them.  If  the  facts  are  insufficient, 
let  him  say  so;  if  they  are  sufficient  to  warrant  a  de- 
cided opinion,  let  him  enunciate  it  clearly,  to  the  end  that 
his  client  may  get  the  maximum  benefit  of  his  investi- 
gations. 

"The  proof  of  the  pudding  is  in  the  eating."     The 

—54— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

exact  profit  to  be  won  from  a  certain  block  of  ore- 
bearing  ground  is  best  known  when  it  has  been  mined 
and  milled;  even  the  most  uniform  ore  has  its  spots  of 
greater  and  lesser  richness;  the  best  estimate  is  there- 
fore only  a  close  approximation  based  upon  the  doctrine 
of  averages.  Nevertheless,  if  such  estimates  are  occasion- 
ally wide  of  the  mark  it  is  not  merely  because  of  la  malice 
des  choses,  that  essential  contrariness  of  things  which  will 
baulk  even  the  best  of  engineers,  but  more  frequently 
it  arises  from  the  disregard  of  the  A,  B,  C  of  proper  pro- 
cedure and  a  judgment  vitiated  by  financial  participation 
in  the  undertaking  itself.  I  am  not  of  those  who  believe 
that  the  sampling  of  a  mine  is  the  one  decisive  factor  in 
the  diagnosis  of  it,  on  the  contrary,  I  hold  that  as  evi- 
dence it  is  crucial  or  merely  collateral,  according  to  the 
circumstances  of  the  case,  which  may  depend  upon  the 
past  record  of  the  mine,  its  future  possibilities,  the  geo- 
logical environment,  economic  conditions  and  other  fac- 
tors of  primary  importance. 

INFERENCES   FROM    SAMPLING. 

When  the  sampling  has  been  thoroughly  done,  the  en- 
gineer is  in  possession  of  many  important  facts.  If  he 
is  a  novice  he  will  have  learned  the  assay-value  of  the 
ore  at  each  of  the  spots  he  has  sampled  and  he  will  have 
learned  little  else. 

Nothing  illustrates  so  well  the  proverb  that  "a  little 
knowledge  is  a  dangerous  thing"  as  the  direct  infer- 
ences from  the  results  of  sampling.  To  emphasize  this 
statement,  I  will  take  an  instance  which  is  founded  on 
fact.  In  Fig.  5  you  have  a  longitudinal  section  of  a 
small  mine,  which  has  been  apparently  well  opened  up. 
The  shaft  is  vertical,  and  does  not  follow  the  vein,  which; 
inclines  to  the  west.  The  levels  to  the  north  have  found 
nothing,  those  to  the  south  (A  B  and  C  D)  have  cut 
through  ore  which  is  also  explored  by  a  raise  (K  L),  and 
two  winzes  (E  F  and  G  H).  At  the  lower  levels,  stopes 
have  been  started  and  the  'backs'  make  an  excellent  show- 
ing, as  the  figures  indicate.    The  latter  are  not  expressed 

—55—  , 


SAiMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 


jfMn  Shaft 


Section  of  Workings  Exhibiting  the  Results  of  Sampling, 

Fig.  5. 

Ore  estimated 


;  I  Section  of  Workings  Showing  t.he  Geological  Features. 

Fig.  6. 


I  i 
!  I 


i   i 


0       5       10  feet 


rrr77TrTTS^Z> 


^M-Mi. 


W^'' 


Granite 


Porphyry 


Ore  Bodies 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

in  any  particular  unit,  but  it  is  to  be  supposed  that  zero 
means  barren,  2^/2  means  pay-ore  and  so  on,  up  to  8, 
which  marks  rich  ore.  The  results  are  indicated  on  the 
section.  The  inference,  as  to  the  amount  of  ore  in  the 
mine,  is  exhibited  by  the  cross-hatching. 

In  arriving  at  this  estimate  the  novice  is  guided  by 
his  sampling  alone ;  he  has  failed  to  take  note  of  the  geo- 
logical features.  It  may  happen  that  the  superintendent 
of  the  mine,  an  old,  practical  miner,  we  will  suppose,  has 
told  him  that  the  ore  is  fairly  uniform  within  the  limits  of 
the  shoot,  and  that  while  the  rock  does  indeed  vary  a 
little  in  places,  this  variation  does  not  seem  to  affect 
the  general  distribution  of  values.  Or,  again,  it  may  be, 
as  happens  often,  that  the  man  in  charge  of  the  mine  is 
honest,  but  not  honorable,  and,  instead  of  committing 
himself  to  such  statements  as  the  foregoing,  he  keeps 
discreetly  silent,  or,  under  cover  of  appearing  to  refrain 
from  influencing  our  young  engineer,^  he  absents  himself 
while  the  sampling  is  in  progress,  and  leaves  his  shift- 
boss  or  some  other  underling  to  do  the  talking.  Per- 
haps, indeed,  those  in  charge  of  the  mine  really  do  not 
appreciate  the  true  condition  of  affairs,  and  the  engineer 
is  deceived  by  an  oversight  which  will  appear  almost  ex- 
cusable. However,  the  mischief  is  done,  the  mine  is 
taken  over  by  a  financial  syndicate  or  a  mining  company. 
Before  the  business  is  actually  consummated,  another 
engineer,  representing  another  financial  interest,  is  sent 
out  to  make  an  examination.  We  will  suppose  that  this 
man  is  of  greater  experience  than  the  last ;  he  is,  more- 
over, familiar  with  the  geological  structure  of  this  par- 
ticular district  or  of  another  similar  to  it.  He  proceeds 
to  sample.  The  accuracy  of  the  previous  sampling  is 
confirmed,  but  in  the  course  of  his  investigation  he  has 
noticed  that  the  country  is  not  uniform,  and  that  there 


ij  have  supposed  the  blunder  to  be  due  to  youthful  inexperience,  for  this 
IS  the  least  blamable.  but  as  a  matter  of  fact,  blunders  of  this  kind  are  due 
to  ignorance  and  are  frequently  made  by  older  men  of  the  kind  described 
as  "thoroughly  practical,"  where  the  particular  emphasis  on  "practical"  is 
Indicative  of  utter  lack  of  training  or  such  technical  education  as  an  engi 
neer  requires. 

—57— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

are  two  different  rocks  exposed  by  the  workings.  One 
is  obviously  granite,  the  other  he  cannot  label  accurately 
without  a  microscopic  section,  but  it  is  evidently  an  erup- 
tive which  has  intruded  into  the  granite ;  so,  on  account 
of  its  speckled  appearance,  he  applies  the  term  'por- 
phyry' to  it,  and  when  the  sampling  is  finished  he  spends 
a  few  days  in  carefully  examining  the  workings  with  a 
view  to  getting  an  idea  of  the  structural  relations  be- 
tween these  two  rocks.  The  results  are  set  down  graph- 
ically, and  he  obtains  the  information  illustrated  in  Fig. 
0.  By  examining  this  section,  side  by  side  with  Fig.  5, 
it  will  be  seen  that  his  inferences  are  warranted.  The 
consequences  are  surprising.  He  discovers  that  there 
is  no  continuous  ore-shoot  from  the  surface  downward, 
but  that  there  are  two  comparatively  small  lenticular 
ore-bodies  which  occur  where  the  vein  cuts  through  the 
porphyry  sheets,  and  only  in  those  portions  of  the  por- 
phyry where  certain  cross-veins  have  exercised  an  en- 
riching effect.  When  the  vein  gets  into  granite  it  be- 
comes poor,  and,  moreover,  it  pinches,  a  fact  which  the 
sampling  did  not  sufficiently  emphasize.  The  ore-reserves 
are  cut  down  to  a  fraction  of  the  previous  estimate, 
and  the  future  prospects  of  the  mine  are  considered 
most  uncertain,  because  the  ore  already  found  is  due 
to  local  structural  conditions  which  are  unlikely  to  recur. 
Another  kind  of  error  may  be  instanced.  It  is  more 
frequent  in  placer  mining  than  in  the  estimation  of  ore 
in  lodes.  Fig.  7  represents  the  cross-section  of  a  deposit 
of  tin  gravel  in  an  old  river  channel  now  capped  with 
basalt.  The  deposit  was  sampled  through  shafts  which 
cut  right  down  to  the  'gutter,'  180  ft.  deep,  below  the 
basalt.  In  this  sampling  the  gravel  was  taken  all  the  way 
down  the  shafts  and  mined  together,  so  that  each  shaft 
yielded  one  large  sample.  From  the  weight  of  this  and 
that  of  the  resulting  grains  of  tin,  after  panning  down, 
the  percentage  was  calculated.  The  estimate  of  quantity 
was  based  upon  the  length  of  channel,  within  the  bound- 
aries of  the  property,  multiplied  into  an  inverted  tri- 
angle, ABC,  the  base  of  which  was  the  top  width  of  the 


SAxMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

deposit  and  the  apex  the  'gutter.'  The  cubic  yards 
thus  obtained  gave  an  approximately  correct'  result  as 
to  quantity  of  material,  but  the  estimated  average  con- 
tent was  all  wrong,  because  the  method  of  calculation 
disregarded  the  fact  that  the  richest  stuff  was  concen- 
trated at  the  lowest  end  of  the  triangle,  and  actually 
formed  a  very  small  proportion,  in  weight,  of  the  whole. 
Subsequently,  the  ground  was  re-sampled.  The  total 
depth  of  i8o  ft.  was,  in  the  case  of  each  shaft,  subdivided 
into  sections  of  30  ft.,  except  the  last  30,  which  was  fur- 
ther divided  into  two  portions,  one  of  20  ft.,  and  a  lowest 


Fig.  7 


Fn  Basalt' 


Gravel 


t'^'f^  Granite 


of  all,  only  10  ft.  thick.  The  first  30  ft.  was  found  to  be 
almost  barren,  the  next  30  ft.  assayed  0.2  per  cent,  of  tin, 
the  next  30  ft.  0.28  per  cent.,  and  so  on,  increasing  grad- 
ually until  the  last  10  ft.  was  reached.  This  was  very 
rich,  1.5  per  cent.,  on  account  of  concentration  in  the 
'gutter'  or  bed  of  the  channel.  Each  layer  was  calcu- 
lated separately  as  to  quantity  and  average  contents,  giv- 
ing results  which  proved  the  bottom  to  be  very  profit- 
able, but  difficult  to  work  on  account  of  want  of  gradient 
for  hydraulicking  operations ;  while  the  uppermost  por- 
tions of  the  deposit  were  found  to  be  too  poor  for  profit, 
but  yet  requiring  handling  in  order  to  get  at  the  lower- 
lying  part  of  the  gravel. 


-59- 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

The  method  first  described  would  have  given  a  very- 
much  exaggerated  idea  of  the  cubic  yardage  and  a  false 
notion  of  a  uniformity  of  value,  besides  ignoring  the 
working  difficulties  to  be  encountered  in  the  extraction 
of  the  tin  from  the  successively  deeper  layers  of  the 
gravel.  This  is  very  much  like  the  sampling  of  a  vein  in 
a  prospect  shaft  where  the  samples  are  taken  every  lo  ft. 
and  are  allowed  to  fall  to  the  bottom  where  they  mingle 
confusedly  and  eventually  form  a  ''large  sample,"  the 
weight  of  which  is  mistakenly  supposed  to  give  assurance 
of  the  accuracy  of  the  average  deduced  from  the  subse- 
quent assay. 

It  is  not  necessary  to  cite  other  instances  of  this  kind. 
Happily,  they  are  rare.  The  real  work  of  good  judgment 
usually  commences  in  the  estimation  of  future  prospects. 
Mines  are  very  rarely  bought  merely  for  the  ore  proved 
up  by  complete  evidence;  the  attractive  feature  is,  as  a 
rule,  a  speculative  enhancement  of  value  likely  to  arise 
from  further  discovery.  This  is  where  the  trouble  begins. 

THE  FUTURE  PROSPfiCTS  OF  A  MINE. 

There  is  room  for  the  exercise  of  a  wonderful  lot  of 
common  sense  in  the  judgment  of  a  mine.  Science,  after 
all,  is,  as  Huxley  himself  said,  ''organized  common 
sense."  If  you  have  been  walking  along  a  road  which 
has  been  straight  and  level  for  five  miles  you  are  un- 
likely to  go  wrong  in  supposing  that  it  will  continue 
straight  and  level  for  another  half  mile,  even  though  the 
foliage  prevents  you  from  seeing  more  than  a  hundred 
yards  ahead,  but  if  you  have  proceeded  along  a  road 
for  a  half  mile  or  so  only,  it  would  be  deemed  foolish  to 
predict  that  the  road  will  maintain  the  same  gradient  and 
direction  for  five  miles  further.  Thus,  in  estimating  the 
persistence  of  an  ore-body  you  may  be  justified  in  count- 
ing upon  its  continuity  for  another  hundred  feet  if  the 
ore  has  already  persisted  with  some  degree  of  uniformity 
for  five  or  six  hundred  feet  downward,  and  so,  on  the 
contrary,  you  will  be  playing  with  Providence  if  you  as- 


SAMPLING   AND    ESTIMATION    OF    ORE    IN    A    MINE 

sume  a  continuation  for  several  hundred  feet  on  the 
part  of  an  ore-shoot  which  has  as  yet  been  traced  for 
only  50  ft.  from  the  surface.  In  this  respect  the  mines 
of  the  Rand  afford  a  striking  contrast  to  most  precious 
metal  mines.  The  'banket,'  in  comparison  with  the  ordi- 
nary type  of  gold-vein,  has  the  persistence  and  uniform- 
ity of  a  coal-seam,  and  explorations  on  a  large  scale  all 
over  a  very  extensive  region  will  warrant  an  engineer  in 
making  assumptions  which  would  be  ludicrous  in  Cali- 
fornia or  Colorado.  Yet,  even  under  these  excepJ:ional 
conditions,  such  general  evidence  of  continuity,  when 
applied  to  an  individual  mine,  must  be  amenable  to  the 
particular  testimony  obtainable  in  that  particular  mine 
or  in  the  workings  of  its  immediate  neighbors.  In  most 
gold-mining  districts  there  is  such  an  absence  of  uni- 
formity in  the  structure  and  behavior  of  gold-bearing 
lodes  that  it  becomes  imperative  to  rely  upon  the  par- 
ticular testimony  afforded  by  each  mine.  Such  testi- 
mony, however,  must  be  read  in  the  broad  light  of  ex- 
perience. For  this  reason  the  estimates  of  ore  likely 
to  be  opened  up  by  further  development  will  be  more 
nearly  correct  on  the  Rand  than  similar  calculations  made 
by  the  same  engineer  elsewhere.  Another  distinction  ob- 
tains ;  a  precious  metal  mine  in  Colorado  or  California,  by 
reason  of  mining  laws  which  give  the  owner  of  the  outcrop 
the  right  to  follow  the  vein  indefinitely  in  depth,  has  a 
future  which  is  not  confined  to  the  narrow  limits  of  mere 
acreage,  such  as  is  imposed  in  the  Transvaal,  in  Australia, 
Mexico,  etc.  This  renders  the  future  of  the  American  mine 
more  speculative,  while  at  the  same  time  it  lays  upon  the 
engineer  the  responsibility  of  appraising  possibilities 
-which  are  quite  beyond  the  Hmits  of  ascertainable  fact. 
It  is  no  solution  of  the  problem  to  say  that  such  possi- 
bilities have  no  assessable  value,  for  to  adopt  this  attitude 
is  to  disregard  the  entire  history  of  precious  metal  min- 
ing in  the  Great  West,  and  the  engineer  who  rests  con- 
tent with  the  evidence  which  is  before  his  nose  will  prove 
but  a  disappointing,  and  often  misleading,  adviser  to  an 
enterprising  client. 

—61— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

In  arriving  at  general  conclusions  concerning  the  per- 
sistence of  an  ore-body  in  a  mine,  the  engineer  may  have 
several  guides,  namely,  the  internal  evidence  afforded  by 
the  behavior  of  the  ore-body  in  the  ground  which  he  has 
examined,  the  collateral  suggestions  afforded  by  the  be- 
havior of  other  ore-bodies  in  the  same  mine,  the  general 
evidence  obtainable  in  other  mines  within  the  same 
region. 

When  the  value  of  a  mine  centers  upon  one  large  ore- 
body,  rather  than  several,  there  is  no  opportunity  for 
inferences  founded  on  similarities  of  behavior.  The 
engineer  is  compelled  to  seek  for  internal  evidence.  If 
the  sampling  has  been  properly  conducted  and  the  re- 
sults have  been  set  down  on  the  longitudinal  section  of 
the  workings,  it  will  be  found  that  the  ore  exhibits  local 
variations  from  which  certain  deductions  are  possible. 
The  relatively  richer  parts  of  the  ore-body  may  be  so  dis- 
tributed as  to  indicate  mere  absence  of  uniformity,  and 
nothing  more,  but,  as  a  rule,  the  indications  will  go  fur- 
ther and  suggest  either  that  the  ore-body  is  becoming 
richer  with  increasing  depth  or  poorer  in  the  same  direc- 
tion, or  that  there  are  successive  zones  of  richer  or 
poorer.  Again,  the  lode  may  be  as  rich  as  heretofore, 
but  a  change  may  be  apparent  either  by  way  of  a  short- 
ening of  the  ore-shoot  or  of  a  narrowing  of  the  pay- 
streak.  For  all  these  possibilities  one  has  to  search  amid 
the  tangled  mass  of  evidence. 

One  or  two  instances  will  serve  to  illustrate  such  pos- 
sibilities. In  the  San  Juan  region  of  Colorado  the  pre- 
vailing geological  formation  is  a  volcanic  breccia,  of 
Tertiary  age,  built  up  of  fragments  of  andesite,  which  are 
arranged  in  neatly  horizontal  layers.  The  veins  cut 
through  this  rock  without,  commonly,  causing  any  very 
big  dislocation.  In  these  gold-bearing  quartz  veins 
there  occur  ore-shoots  having  a  pitch  which  is  usually 
not  far  from  the  vertical,  and  while  such  ore-shoots  may 
be  worked  out  in  their  entirety  so  as  to  exhibit  an  un- 
broken continuity  in  the  stoping,  nevertheless,  to  those 
who  are  observant  of  the  variations  in  the  grade  of  the 

—62— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

ore,  as  recorded  by  daily  assays  or  weekly  mill  returns, 
it  is  very  clear  that  such  variations  coincide  with  the 
changes  in  the  country;  that  is  to  say,  the  ore-body  will 
be  characterized  by  nearly  horizontal  bars  of  enrichment 
which  are  traceable  to  the  effect  produced  upon  the  vein 
by  the  particular  layer  of  breccia  through  which  it  is 
passing  at  a  particular  horizon.  My  observations  lead 
me  to  suppose  that  the  composition  of  certain  layers  of 
the  fragmental  volcanic  rock  is  responsible  for  the  effects 
noticed,  and  that  the  layers  which  have  the  finest  texture 
are  those  which  are  the  most  beneficial,  for  reasons  out- 
side of  the  present  discussion.  One  example  will  suf- 
fice. In  Fig.  8  it  is  seen  that  a  series  of  adits  penetrate 
a  steep  mountain  which  is  entirely  composed  of  andesite 
breccia.  The  lowermost  adit  goes  through  an  average 
of  2^  ft.  of  6  dwts.  of  gold,  which  is,  in  the  district 
where  the  mine  is  situated,  rather  low-grade.  The  first 
portion  of  the  level  and  the  end  of  it  pass  through  barren 
ground,  so  that  there  is  some  evidence  of  the  exist- 
ence of  an  ore-shoot  of  this  low-grade  stuff.  In  the 
stopes  there  is  no  change  to  record  until  the  next  level 
overhead.  No.  2,  is  approached.  A  raise  connecting  with 
this  level  passes  through  8  dwt.  ore.  No.  2  adit  makes 
a  much  better  showing,  and  averages,  fairly  uniformly, 
II  dwts.  for  an  average  width  of  4  ft.  Here  also  the 
level  passes  out  of  ore  at  a  point  nearly  vertically  over 
the  corresponding  impoverishment  at  No.  i.  The  stopes 
above  No.  2  give,  at  first,  results  as  good  as  the  drift, 
but  in  going  upward  they  show  a  marked  falling  off, 
so  as  to  average  one-half  the  results  given  by  the  No.  2 
level.  At  No.  3  the  average  of  the  level  is  only  3)^  ft. 
of  4-dwt.  ore;  this  is  doubtless  the  reason  why  the  drift 
was  not  extended  to  the  limit  of  the  sh'oot.  A  raise  put 
up  from  this  level  is  in  poor  ground  until  within  50  ft. 
of  No.  4;  then  marked  improvement  sets  in.  This  is 
confirmed  by  the  results  from  the  No.  4  adit,  which  gives 
an  average  of  3  ft.  of  lo-dwt.  ore.  This  good  mill-stuff, 
however,  does  not  extend  much  above  the  level,  as  is 
proved  by  a  raise,  which  gets  into  poor  ground. 

—63- 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 


Fig.  8. 
fiHO\nxG  Results  of  Sampling. 


No.3^ 


No.2 


^      .     ^y     An  average  of  3  ^  ft.  of  10  dvi-t.  ore        y 


'A  Djstinct  improvement  ( S, 

r  ""    u 

S.B  average  of  3"-;  ft.orFjwt.  cTr^T^s— 


broi>e>  a%f  latrfcu  >''  dwt-.  but  inij'roveJ  in  sc'; 
An  a\>-icigt-  of  2--.  ft.  r,t  li  d-.i'T.  tat  aloTiy  This 


W)».)»»»»y»}}j>j/}»}}W)}}}mY/j 


Fig.  9 
Showikg  Rock  Structure 


JTo.  1 


—64— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

These  data  are  perplexing  and  indicate  an  alternation 
of  values  which  renders  any  estimate  very  hazardous. 
When,  however,  the  geological  structure  is  investigated, 
there  is  found  to  be  a  relation,  between  values  and  struc- 
ture, which  illuminates  the  entire  problem.  In  Fig.  ii 
there  is  given  a  section,  on  the  plane  of  the  vein,  which 
exhibits  the  position  of  the  series  of  layers  through 
which  the  vein  cuts.  This  helps  to  explain  the  results 
shown  in  Fig.  9.  The  zones  of  enrichment  penetrated 
by  adits  No.  i  and  No.  4  coincide  with  two  layers  of 
close-textured  tuff,  or  fine-grained  breccia,  laid  down 
during  the  intermittent  violence  of  volcanic  eruption. 
The  intermediate,  overlying,  and  underlying  beds  of 
breccia  are  coarse,  and,  for  some  reason  which  does  not 
concern  the  present  inquiry,  they  are  less  favorable  to 
ore-deposition  than  the  occasional  layers  of  tuff. 

In  this  instance  the  evidence  was  very  clear;  in  other 
mines  situated  in  the  same  region  it  is  not  practicable, 
on  account  of  the  decomposition  of  the  enclosing  country, 
to  establish  the  relationship  so  clearly,  but  it  is  a  matter 
of  experience  that  notable  variation  in  both  the  width 
and  value  of  the  lodes  does  occur  in  a  similar  nearly 
horizontal  direction.  The  understanding  of  it  is,  there- 
fore, a  clue  to  many  apparent  vagaries  in  the  distribution 
of  rich  ore. 

In  the  case  of  a  mine  which  contains  several  ore- 
bodies  there  are  many  useful  suggestions  to  be  obtained 
from  a  comparison  of  their  characteristics.  Careful 
investigation  will  lead  sometimes  to  the  detection  of 
peculiarities  applicable  to  the  general  occurrence  of  ore 
in  that  particular  mine.  The  longitudinal  section  of  the 
stopes  warrants  the  close  study  given  by  a  general  to  the 
map  of  the  region  which  is  to  be  the  scene  of  his  cam- 
paign. Stope-maps  are  not  always  correct ;  sometimes 
they  are  incorrect  to  a  very  misleading  degree.  A  few 
measurements,  at  least,  should  be  made  to  test  this 
point,  and  occasionally,  when  the  general  accuracy  of 
the  surveys  warrants  a  doubt,  it  is  a  good  precaution  to 
make  one's  own  map  of  the  mine,  either  personally,  or 


SAMPLING   AND    ESTIMATION    OF    ORE   IN^  A   MINE 

by  engaging  the  services  of  a  reliable  surveyor.  The 
conditions  are  so  diverse  that  it  is  impossible  to  give  a 
typical  illustration,  but,  by  way  of  suggestion,  I  will 
append  an  example  of  inferences  derived  from  a  study  of 
the  old  workings  of  a  mine.  It  is  a  small  mine ;  for, 
after  all,  the  most  perplexing  of  problems  is  a  'pros- 
pect,' and  it  is  found  in  practice  that  the  greatest  mis- 
takes and  the  exercise  of  the  keenest  judgment  are  alike 
exhibited  in  the  appraising  of  the  uncertainties  of  an 
undeveloped  mine ;  therefore,  the  size  of  the  property 
chosen  as  an  illustration  scarcely  requires  apology. 


Prospect 
Shaft  X 


Section  of  a  Mine 
Showing  Ore 
IN  Chimneys 


Sloped 


VW\  Rich  Ore 


Fig.  10 


Fig.  10  represents  a  section  of  the  workings.  These 
consist  of  two  adits,  on  the  vein,  together  with  a  dis- 
covery shaft,  which  joins  the  upper  level  (G  H),  and  a 
raise  connecting  the  latter  with  the  lower  level  (K  M). 
The  problem  is  to  determine  what  weight  should  be 
given  to  the  occurrence  of  a  good  width  (say,  4  ft.)  of 
very  rich  ore  (say,  5  oz.  of  gold  per  ton)  in  the  ends 
(H,  M)  of  the  two  levels.  In  such  a  case  it  is  most  im- 
portant to  investigate,  with  thoroughness,  the  mode  of 
occurrence  of  the  ore-bodies  previously  encountered  in 


-66- 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

the  mine.  This  is  not  always  easy ;  old  workings  have 
been  allowed  to  cave  or  the  ground  is  heavy,  and  they 
are  carefully  timbered  so  as  to  impede  examination. 
Sometimes  this  is  intentional.  In  the  case  cited  the 
upper  level  appears  to  carry  one  continuous  body  of 
ore  from  P  to  the  breast  at  H,  with  a  small  compara- 
tively barren  interval  S.  T.  The  timbering  obscures  the 
fact  that  the  first  half  of  the  stope  P  S  is  only  a  cutting- 
out  stope,  which  was  worked  for  the  purpose  of  testing 
a  low-grade  portion  of  the  vein;  the  same  applies  to 
more  than  two-thirds  of  the  stope  V  W,  at  the  lower 
level.  Similarly,  while  the  timbering  between  T  and  Z 
indicates  a  good  length  of  stope,  as  a  matter  of  fact  the 
innermost  half  of  this  is  little  more  than  a  'cutting-out 
stope'  in  comparatively  poor  ground.  The  truth  is, 
the  ore-body  A  B  did  not  go  to  surface,  having  been  first 
cut  by  the  discovery  shaft  at  70  ft.  down;  it  is  a  very 
short  body  of  ore,  almost  a  chimney,  and  it  diminishes 
in  approaching  the  lower  level ;  further,  the  ore-body  C 
D  is  still  less  persistent ;  it  is  a  narrow,  short  lens  which 
does  not  hold  out  to  the  lower  level,  nor  does  it  go  up 
more  than  half-way  to  surface,  but  it  makes  a  sort  of 
spurious  connection,  by  means  of  the  raise,  with  another 
patch  of  ore  E,  on  the  second  level.  In  practice,  such 
workings  as  these  are  largely  inaccessible  and  usually 
dangerous ;  as  a  consequence,  the  statements  of  an  m- 
accurate  superintendent,  who  desires  to  serve  h'is  em- 
ployer by  putting  things  in  the  most  favorable  light,  are 
apt  to  carry  more  weight  than  they  should,  so  that,  with 
the  usual  desire  for  quick  decision  on  the  part  of  both 
the  vendor  and  the  vendee,  the  engineer  may  be  pushed 
to  a  conclusion  which  the  actual  facts  do  not  warrant.  It 
may  seem  to  one  who  has  missed  the  essential  character 
of  these  ore-bodies,  through  the  failure  to  examine  the 
old  workings,  that  the  good  width  of  rich  ore  at  H  and 
at  M  indicates  the  beginnings  of  a  big  body  of  ore 
which  persists,  at  least,  from  one  level  to  the  other;  but 
to  another  man,  who  has  carefully  studied  the  old  work- 
ings, it  is  evident  that  the  probabilities  point  merely  to 

—67— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 


Plan 

showing  sampxinq 

1^^^  Supposed  Ore 

Fig.  11 


Plan 
showing  lode  structure 

i-:i;-}^-^'-^  ■  ■'.]  Ore  Lenses 

Fig.  12 


SAMPLING    AM)    ESTIMATION'    OF    ORE    IN    A    MINE 

another  narrow  lens  of  uncertain  extent.  The  moral  is, 
don't  allow  yourself  to  be  hurried  into  an  opinion  by 
either  party  to  the  transaction,  and  beware  of  workings 
which  are  stated  to  be  inaccessible,  because  such  inac- 
cessibility may  have  a  purpose. 

Another  illustration  will  emphasize  the  warning-  con- 
veyed in  the  last  sentence.  In  Fig.  ii  is  represented  the 
single  level  of  a  California  mine  which  is  reached  by 
the  western  cross-cut  shown  to  the  left  of  the  drawing. 
The  level  is  450  ft.  long;  owing  to  the  fact  that  the  coun- 
try is  a  soft  black  slate  the  ground  is  very  heavy,  and 
the  level  is  closely  timbered  throughout,  permitting 
only  occasional  glimpses  of  ore,  at  the  points  where  short 
cross-cuts  have  been  run  out  eastward.  At  these  places, 
D  E,  F  G,  B  C  and  H  K,  the  lode  is  plainly  visible,  and 
exhibits  from  6  to  8  ft.  of  fairly  uniform  quartz  carry- 
ing an  average  of  from  6  to  7  dwt.  of  gold,  in  a  district 
where  working  costs  are  about  $3  per  ton.  After  sam- 
pling these  cross-cuts,  examining  the  little  there  was  to 
be  seen,  and  finding  that  the  ground  really  seemed  heavy 
enough  to  justify  the  close  timbering  over  the  level,  the 
engineer  came  to  the  conclusion  that  he  was  dealing 
with  one  of  those  persistent  ore-bodies  not  uncommon 
in  that  part  of  California,  and  he  estimated  the  ore  to 
have  the  continuity  exhibited  by  the  cross-hatching  in 
the  drawing.  However,  he  had  wholly  misunderstood 
the  real  character  of  the  ore-occurrence,  as  Fig.  12  will 
indicate.  The  close  timbering  was  not  necessary  in 
order  to  sustain  the  ground  alone ;  it  was  an  excuse  for 
obscuring  certain  facts.  The  quartz  exposed  by  the  sev- 
eral cross-cuts  belonged  to  a  number  of  small  lenses. 
These  abutted  against  the  "fissure,"  or  main  line  of 
fracture,  which  constituted  the  supposed  lode ;  the  lenses 
were  short-lived  and  lay  with  their  longer  axis  parallel 
to  the  cleavage  of  the  slate ;  the  body  at  D  E  only  lasted 
as  far  as  two  sets  of  timber ;  that  of  F  G  died  out  at  60 
ft.,  and  that  at  B  C  "petered  out"  at  40  ft.  above  the 
level,  while  the  last  one,  H  K,  went  up  as  a  narrow  body 
for  a  distance  of  160  ft.     Compare  Fig.  11  with  Fig.  12 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

and  quote  Hamlet's  comment  on  his  own  portrayal  of 
his  mother's  successive  husbands.  This  incident  did  not 
befall  a  tyro,  but  an  old  campaigner  who  was  caught 
by  his  own  carelessness  under  circumstances  from  which 
only  the  greatest  wariness  could  have  saved  him. 

Misconceptions  concerning  the  conditions  contribu- 
tory to  the  localization  of  ore-shoots  and  the  conse- 
quent mistaken  ideas  as  to  the  future  possibilities  of  a 
mine  have  often  arisen  from  a  misunderstanding  of  vein 
intersections.  In  the  case  of  a  well-known  mine  in  south- 
western Colorado  a  length  of  2,700  ft.  of  ore  had  been 
opened  up  by  an  upper  level,  and  stopes,  which  had 


JWinzel43Ft.  Deep 


Stoped 


0    100  200 


Longitudinal  Section        Fig.  13 


worked  out  most  of  the  ore  except  at  one  end,  and  had 
proved  that  it  persisted  to  the  surface,  an  average  dis- 
tance of  150  ft.  above  this  upper  level.  In  the  mean- 
time two  winzes  had  demonstrated  the  apparent  down- 
ward continuance  of  the  ore-body.  A  longitudinal  sec- 
tion of  these  workings  is  shown  in  Fig.  13.  In  the  ex- 
amination of  the  mine  by  several  distinguished  mining 
engineers,  it  was  assumed  that  the  winzes,  one  of  which 
was  58  ft.  and  the  other  143  ft.  deep,  proved  the  con- 
tinuity of  the  ore-body  to  an  extent  sufficient  to  warrant 
a  price  for  the  property  which  was  a  good  deal  in  ex- 
cess of  the  net  profits  to  be  won  from  the  ground  as 
measured  to  the  bottom  of  the  winzes.  In  the  sequel 
it  was  found,  after  the  mine  had  been  purchased,  and 
when  deeper  workings  had  explored  the  lower  horizon, 
that  the  ore-body  owed  its  existence,  and  certainly  its 
shape  and  position,  to  the  intersection  with  another  lode. 
This  made  a  "scissors  crossing,"  the  hinge  of  which  was 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

a  line,  dipping  west  at  an  angle  of  7°  30'.  The  ore-body 
reached  from  this  line  to  the  surface,  while  downward  it 
extended  about  a  hundred  feet  or  so.  Other  scattered 
smaller  bodies  of  ore  were,  it  is  true,  encountered,  but 
the  main  ore-body  of  the  mine,  upon  which  its  value 
was  based,  extended  only  about  a  hundred  feet  below 
the  broken  line  shown  across  the  longitudinal  section  in 
Fig.  13.  Such  an  instance  as  this  proves  the  need  for 
deciphering  the  conditions  which  have  affected  the  dis- 
tribution of  ore.  The  crossing  referred  to  was  visible 
in  the  stopes  at  the  east  end,  but  only  to  an  inquiring 
observer  who  suspected  some  eccentricity  of  lode  struc- 
ture. 

COLLATERAL  EVIDENCE. 

Of  all  the  collateral  evidence  likely  to  aid  the  engineer 
in  a  correct  diagnosis  of  the  condition  of  the  mine  he  is 
examining,  none  is  so  useful  as  that  derived  from  a  study 
of  the  surrounding  district.  Mines  situated  in  the  same 
district  are  apt,  on  account  of  an  identical  geological  en- 
vironment, to  exhibit  similar  symptoms  and  to  suggest 
like  possibilities.  The  general  study  of  a  region,  there- 
fore, is  a  useful  preliminary  to  the  investigation  of  a 
particular  mine.  Moreover,  there  is,  I  am  glad  to  say, 
a  certain  spirit  of  camaraderie  among  professional  men 
of  the  right  sort  which  will  enable  a  stranger  to  obtain 
many  useful  hints  from  resident  engineers,  if  he  is  prop- 
erly made  known  to  them. 

The  characteristics  of  ore-deposits  within  limited  areas 
are  often  well  marked;  in  one  case  persistence,  without 
probabilities  of  either  bonanzas  or  borrascas,  is  the  rule,  as 
on  the  Rand ;  in  another  case  an  irregular  distribution  of 
values  is  to  be  expected,  as  in  most  silver-lead  deposits  in 
limestone,  and  also  in  many  rich  telluride  veins  where  sec- 
ondary enrichment  has  been  at  work ;  again,  certain  lines 
of  faulting  and  well-defined  contacts  between  sedimentary 
and  eruptive  rocks  may  be  recognizable  as  factors  in  deter- 
mining the  place  of  rich  bodies  of  ore,  as  is  illustrated  by 
the  Aspen  and  Rico  districts  in  Colorado. 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 


Another  very  important  factor,  in  influencing  the  rela- 
tive richness  of  a  lode  in  depth,  is  the  position  of  the 
ground-water  level.  No  engineer  can  afford  to  disre- 
gard it.  Two  opposite  cases  may  be  cited.  In  certain 
arid  regions  of  the  southwestern  part  of  North  America, 
which  it  would  be  invidious  to  specify  further,  the  per- 
sistence of  rich  ores  coincides  roughly  with  the  zone  of 
oxidation.  I  recall  a  well-known  mine  which,  at  the  time 
it  was  examined  by  several  reputable  engineers,  exposed 
an  ore-body  nearly  i,8oo  ft.  in  length.  This  was  the 
length  of  the  ore  traversed  by  the  longest  level,  which 
was  also  the  deepest.  (See  Fig.  14.)  Both  ends  of  this 
level  were,  however,  still  in  ore.    Three  winzes  proved 


100  200  Feet 

Fig.  14 

the  ore  for  a  further  depth  of  about  30  ft.  The  upper 
workings  contained  carbonates  of  lead  and  iron-stained 
quartz  rich  in  the  precious  metals.  At  the  third  level 
there  was  evidence  of  the  edge  of  the  oxidized  zone,  for 
sulphides  predominated.  All  of  the  winzes  left  off  in  sul- 
phides, of  average  tenor  as  regards  gold  and  silver,  but 
exhibiting  a  noteworthy  admixture  of  zinc-blende  amid 
the  galena.  One  engineer  who  examined  the  mine  allowed 
50  ft.  below  the  lowest  level  as  ore  to  be  considered 
in  appraising  the  value  of  the  property,  but  he  allowed 
nothing  beyond  the  ends  of  the  level.  This  was  adversely 
criticized.  However,  among  the  reasons  prompting 
a  conservative  attitude  was  the  recognition  of  the  fact 
that  many  gold  and  silver  veins  in  that  region  turned 
into  poor  zinc-bearing  lodes  at  a  short  distance  below 
the  limit  of  visible  oxidation.  In  the  sequel  he  was 
proved  to  be  right.    When,  later  on,  the  mine  underwent 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

extensive  development,  it  was  found  that  at  45  ft.  below 
the  lowest  level  the  ore  became  poor  in  gold  and  silver, 
but  heavily  charged  with  zinc-blende,  while  at  the  same 
time  the  lowest  level  ran  out  of  the  ore-body  within  a 
few  feet  at  both  ends.  In  this  regard  it  is  a  curious  fact 
how  often  good  miners,  especially  if  they  are  selling  a 
mine,  stop  their  explorations  at  the  proper  psychologi- 
cal moment. 

The  great  copper  region  around  Butte,  in  Montana, 
exemplifies  another  aspect  of  the  inquiry.  Very  exten- 
sive exploration,  prompted  by  the  extreme  richness  of 
the  lodes,  has  permitted  of  the  accumulation  of  evidence 
which  proves  conclusively  that  the  big  ore-bodies  are 
the  result  of  a  secondary  enrichment  brought  about  by 
the  leaching  of  copper  by  the  ground-water  and  its  pre- 
cipitation upon  the  sulphides  of  the  deeper  zone.  The 
original  vein  stone  consisted  of  iron  pyrite,  copper 
pyrite  and,  probably,  enargite,  while  the  bonanzas  con- 
sist of  the  higher  sulphides,  bornite,  covellite  and  chal- 
cocite.  Experiments  have  been  made^  in  the  laboratory 
which  illustrate  the  formation  of  chalcocite  (copper 
glance)  on  pyrite  by  precipitation  from  a  solution  con- 
taining copper  sulphate,  such  as  would  be  the  product 
of  oxidation  through  the  agency  of  the  ground-water. 
As  a  result  of  such  reactions  the  outcrops  are  poor  in 
copper  and  silver,  because  these  have  been  leached  out; 
the  silver  appears  at  a  comparatively  shallow  depth 
(three  or  four  hundred  feet  below  the  surface)  so  as  to 
form  a  zone  of  maximum  enrichment  in  that  metal,  while 
the  copper  has  been  deposited  deeper  still,  so  as  to  form 
the  magnificent  masses  of  high-percentage  ores  which 
have  made  Butte  so  productive.  Below  these  come  the 
first-formed  ores,  chiefly  pyrite  with  a  little  copper 
pyrite,  without  the  higher  copper  sulphides  which  h'ave 
enriched  the  overlying  zone.  While,  therefore,  the  lower 
limit  of  the  horizon  of  secondary  enrichment  has  been 
passed  through  in  some  of  the  mines,  it  must  be  added 


i"The  Synthesis  of  Chalcocite,"   by  H.    V.    Winchell.     Engineering   and 
Mining  Journal,   May  23,   1903. 


7Z- 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

that  in  others  the  rich  sulphides  extend  to  a  depth  of 
over  2,000  ft.  from  the  surface,  so  that  the  effects  of  the 
ground-water  circulation  continue  to  a  profundity  quite 
exceptional  in  the  experience  of  mining.  This  may  be 
due  to  the  intensity  of  the  fracturing  which  followed  the 
deposition  of  the  earlier  sulphides,  and  it  may  be  partly 
a  result  from  the  open  character  of  the  lode  channels 
which  has  permitted  of  the  far-reaching  penetration  of 
the  ground-water,  but,  whatever  the  cause,  it  renders  the 
problem  of  ore-occurrence  at  Butte  quite  exceptional, 
particularly  when  compared  to  cases  such  as  the  one 
quoted  in  the  preceding  instance,  in  Fig.  14.  A  novice 
who  has  sampled  the  shallow  workings  of  a  young  mine 
in  a  region  such  as  Butte  would  be  likely  to  be  misled 
by  the  low  percentage  of  copper,  and  therefore  enunciate 
an  unfavorable  opinion,  whereas  one  who  recognized 
the  effects  of  ground-water  leaching  and  was  guided  pos- 
sibly by  a  knowledge  of  the  changes  encountered  at 
greater  depth,  in  a  neighboring  mine,  would  make  quite 
a  different  inference.  Under  other  circumstances  in  an- 
other region  the  occurrence  of  the  higher  sulphides, 
such  as  covellite  and  chalcocite,  would  suggest  second- 
ary enrichment  and  warn  the  engineer  against  counting 
upon  a  persistence  of  conditions  which  are  frequently 
quite  local. 

Considerations  such  as  these  suggest  very  forcibly 
that  keen  observation  and  wide  experience  are  required 
for  the  appraisement  of  the  potentialities  of  a  prospect. 


CONCLUSION. 

This  discussion  of  the  subject  has  emphasized  one 
great  factor  in  work  of  this  kind,  and  that  is — the  per- 
sonal equation.  No  two  men  set  to  work  in  exactly  the 
same  way,  and  no  two  men  make  exactly  the  same  deduc- 
tions from  a  complicated  series  of  data.  My  views  on 
the  subject  are  those  based  upon  my  own  experience — 
hence  the  inevitable  limitations  of  my  presentation  of  it. 
For  this  reason,  also,  I  have  omitted  to  refer  to  the  litera- 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

ture  which  deals  with  sampling  and  estimates  of  ore,  be- 
cause it  would  have  extended  this  contribution  to  un- 
comfortable length,  had  I  taken  up  the  other  aspects  of 
the  inquiry  as  discussed  by  other  men.  Herein  lies  my 
excuse  and  apology  to  the  Institution  of  Mining  and 
Metallurgy,  to  Messrs.  J.  D.  Kendall,  E.  B.  Kirby,  A.  G. 
Charleton,  W.  Wybergh,  G.  A.  Denny,  S.  J.  Truscott, 
D.  W.  Brunton,  W.  McDermott  and  other  authoritative 
writers  on  the  same  subject. 

The  conclusion  of  the  whole  matter  is  sufficiently 
obvious — so  obvious  as  to  need  but  little  further  in- 
sistence. If  the  discussion  of  the  methods  to  be  applied 
in  the  sampling  and  estimation  of  ore  has  served  to  ac- 
centuate the  difficulties  to  be  encountered  in  the  diag- 
nosis of  a  mine,  then  it  will  have  fulfilled  one  of  the 
purposes  of  this  contribution.  ''Chi  va  piano,  va  sano." 
Extreme  care  is  necessary  at  every  stage  of  the  work — 
care  and  experience,  the  experience  born  of  wide  knowl- 
edge and  practical  work,  without  which  a  man  in  a  mine 
is  no  better  than  the  proverbial  bull  in  a  china  shop, 
with  a  strong  suggestion  of  a  coming  smash ! 

The  old  hands — the  engineers  who  have  sampled 
mines  from  China  to  Peru — do  not  need  my  maxims  to 
guide  them.  They  have  lost  the  cheerful  confidence  of 
their  apprentice  days  and  are  saturated  with  the  experi- 
ence of  difficulty  and  doubt  which  have  been  frankly 
faced  on  a  hundred  occasions.  To  the  youngster,  who 
still  has  to  receive  his  first  fall  and  to  realize  that  few 
data  are  absolutely  certain,  also  that  data  carelessly  ob- 
tained are  inevitably  uncertain — to  him  this  analysis  of 
methods  of  work  will,  it  is  earnestly  hoped,  serve  as  a 
red  flag  of  warning.  To  the  general  reader  who  is  usual- 
ly more  in  sympathy  with'  the  pains  of  the  unfortunate 
investor  in  mines  than  he  is  with  the  causes  which  may 
vitiate  good  advice  from  the  expert,  to  him  these  obiter 
dicta  will,  I  trust,  suggest  forcibly  that  good  advice  is 
hard  to  get  and  expensive — yet  cheap  indeed  compared 
to  thte  costly  experience  of  entrusting  onerous  profes- 
sional duties  to  the  inefficient. 

—75— 


"ORE  DEVELOPED,"  A  DEFINITION.* 

By  Philip  Argall. 

The  phrase  *'Ore  in  Sight,"  so  commonly  used  as  an 
alleged  measure  of  value  in  mine  reports,  is,  in  my  opin- 
ion, a  very  poor  expression.  Taken  literally,  as  the  non- 
professional man  is  most  likely  to  understand  it,  one 
cannot  defend  its  use  except  as  a  misleading,  if  not  de- 
ceptive, euphony. 

The  only  *'ore  in  sight''  in  or  about  a  mining  property 
is  that  broken  and  displayed  at  surface  in  the  ore-bins,  or 
in  dumps.  Perhaps  I  should  also  include  ore  broken  or 
visible  in  the  underground  workings ;  but  this  is  not  what 
is  usually  meant  by  "ore  in  sight"  in  mine  reports. 

I  believe  the  majority  of  mining  engineers  call  vein- 
matter  blocked  out  and  accessible  for  measuring  and 
sampling  on  all  four  sides,  or  at  the  very  least  on  three 
sides,  as  "ore  in  sight,"  while  strictly  speaking  it  is  only 
the  four  planes  bounding  the  block  that  are  visible  to  the 
physical  eye.  The  "mind's  eye"  should  have  no  place 
in  such  an  estimate. 

I  take  it  that  the  whole  purpose  of  sinking  shafts  and 
winzes,  running  cross-cuts,  levels  and  raises,  is,  first,  to 
open  up  and  develop  ore ;  and,  secondly,  to  provide  ready 
means  to  extract  and  market  this  ore  when  developed. 
Once  a  given  piece  of  vein-matter  is  blocked  out  by,  say, 
two  drifts,  150  ft.  apart,  and  two  winzes,  200  ft.  apart,  it 
has  reached  its  highest  state  of  development,  that  is,  it  is 
ready  for  removal  by  stoping;  and  it  is  also  in  the  best 
form  for  sampling  and  measurement,  in  order  to  obtain 
a  close  estimate  of  its  commercial  value.  I  would  sub- 
mit, then,  that  such  a  block  had  best  be  called  "ore  de- 


*From  The  Engineering  and  Mining  Journal  of  February  14, 1903. 
—76— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

veloped"  and  not  "ore  in  sight."  The  former  phrase 
would  correctly  describe  what  it  is,  while  the  latter  just 
as  clearly  shows  what  it  is  not. 

"Ore  developed"  should  in  every  case  be  accessible 
for  measurement,  sampling  and  full  examination  on  all 


O^e'  s//oar: 


sides.*  Block  A,  on  the  diagram  attached,  showing  four 
exposed  sides,  is  perhaps  the  more  usual  form  met  with 
in  a  developed  mine.  Block  B,  however,  with  three  ex- 
posed sides,  also  comes  under  our  classification  of  "ore 


*The  planes  called  foot  and  hanging  walls  are  not  considered  as  sides. 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

developed,"  as  would  also  any  irregularly  shaped  ore- 
body  visible  and  fully  accessible,  on  and  along  all  por- 
tions of  its  perimeter. 

Inasmuch  as  "ore  developed"  forms  the  subject  of  the 
highest  possible,  or  most  accurate,  estimate  engineers  can 
make  of  ore  in  place,  it  should  be  hedged  in  with  the  in- 
variable and  unalterable  requirement,  exposed  on  all 
sides  up  to  the  full  thickness  of  the  ore.  Under  this  con- 
dition the  estimates  of  independent  examining  engineers 
sh'ould  agree  within  the  limits  of  experimental  error.  As 
a  general  rule,  one  is  safe  in  stating  that  the  smaller  the 
blocks  of  "developed  ore"  the  more  accurately  can  their 
contents  be  estimated.  These  blocks,  then,  should  be 
of  moderate  dimensions.  In  ordinary  vein  mining  the 
levels  are  usually  placed  at  intervals  of  lOO  to  150  ft. 
apart,  as  measured  along  the  dip  of  the  vein.  I  would 
suggest  as  a  safe  rule,  within  these  limits,  that  the  length 
of  such  blocks  of  "ore  developed"  should  not  exceed 
twice  their  depth. 

It  may  be  suggested  that  it  is  not  always  imperative  to 
completely  block  out  ore-ground  before  stoping.  Quite 
true.  The  parties,  however,  who  are  developing  their 
mines  for  the  purpose  of  selling  them,  would  be  well  ad- 
vised to  do  so.  Otherwise  the  ore  partially  blocked  out, 
and  in  various  minor  stages  of  development,  must  receive 
separate  and  lower  classifications. 

For  blocks  of  partially  developed  vein-material,  hid- 
den on  one  or  more  sides,  I  would  suggest  the  phrase 
"ore  being  developed,"  or,  for  shortness,  "ore  develop- 
ing." Two  levels,  say,  150  ft.  apart,  simultaneously  pene- 
trating an  ore-shoot,  are,  as  they  advance,  daily  increas- 
ing the  amount  of  "ore  being  developed,"  which,  on  fur- 
ther opening  up  by  winzes  or  raises  connecting  these 
levels  and  cutting  the  ground  into  blocks,  becomes  "ore 
developed." 

"Ore  being  developed"  naturally  falls  into  three 
classes:  First,  where  one  side  only  is  hidden;  second, 
where  two  sides  are  hidden;  third,  where  three  sides 
are  hidden. 


SAMPLING    AND    ESTIMATION    OF    OKK    IN    A    MINE 

As  an  example  of  the  first  class,  we  will  take  block  C, 
which  on  a  casual  glance  might  appear  just  as  safe  an 
estimate  as  block  B.  On  closer  examination,  however, 
it  will  be  noticed  that  on  the  hidden  side  we  can  neither 
determine  the  assay  value  of  the  ore,  its  thickness,  nor  its 
actual  termination.  For  example,  if  the  shoot  follows  the 
heavy  line,  as  one  naturally  assumes  it  would,  we  have 
data  for  a  certain  tonnage,  while  if  it  follows  the  dotted 
line,  we  have  an  increased  tonnage.  Similar  remarks  ap- 
ply to  block  D,  showing  h'ow  the  assumed  tonnage  might 
be  decreased. 

It  is  therefore  abundantly  clear  that  a  given  block  of 
ground  having  one  hidden  side  cannot  be  estimated  as 
closely  as  one  having  all  sides  exposed,  inasmuch  as  in 
the  former  about  25  per  cent  of  the  necessary  data  is  un- 
obtainable. We  are  therefore  fully  justified  in  excluding 
the  block  having  one  side  hidden  from  an  estimate  of 
"ore  developed,"  or,  if  you  please,  "ore  in  sight." 

Again  referring  to  the  diagram,  block  E,  with  two  sides 
hidden,  cannot  in  the  very  nature  of  things  be  estimated 
as  closely  as  blocks  C  and  D,  hence  we  class  it  "ore  de- 
veloping," second  class,  while  block  F,  with  three  sides 
hidden,  very  naturally  falls  into  the  third  class  estimate 
of  "ore  being  developed." 

We  next  come  to  the  future  value  of  the  ore-shoot.  In 
other  words,  to  what  depth  will  the  pay-ore  extend  ?  We 
assume  the  ore-shoot  is  of  fairly  uniform  dimensions  and 
value ;  that  it  continued  so  to  the  lowest  point  of  explora- 
tion,  the  450-ft.  level  on  our  diagram.  The  experience  in 
the  district  went  to  show  thiat  the  horizon  of  pay-ore 
extended  to  1,000  ft.  In  this  case  the  ore  is  not  even 
in  a  stage  of  "ore  developing" ;  it  may  be  entirely  in- 
visible, yet  one  has  reasonable  assurance  from  the  appear- 
ance of  the  vein  and  ore-shoot,  from  the  experience  ob- 
tained in  adjoining  properties,  and  from  other  physical 
phenomena,  that  pay-ore  exists  in  this  shoot  to  a  depth 
of  at  least  900  ft. ;  consequently  the  engineer,  after  mak- 
ing such  allowances  as  he  deems  safe,  looks  forward  with 
confidence  and  reasonable  assurance  that  the  shoot  of 

—79— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

pay-ore  will  extend  to  some  considerable  depth,  perhaps 
to  1,000  ft.  This  I  class  as  *'ore  expectant."  It  deals 
rather  with  the  future  than  with  the  present,  and  more  in 
the  region  of  the  imagination  than  of  fact,  but  is,  never- 
theless, an  important  factor  in  every  mine  valuation,  deal- 
ing as  it  does  with  the  probable  life  of  the  property  as  a 
producing  mine. 

"Ore  expectant"  is  perhaps  the  most  difficult  estimate, 
if  estimate  it  can  be  called,  that  engineers  are  compelled 
to  make,  because  of  the  few  facts  available;  hence  it  is 
only  to  be  supposed  that  opinions  and  estimates  of  dif- 
ferent engineers  must  widely  vary.  While,  therefore, 
with  "ore  developed"  the  highest  and  most  accurate  esti- 
mate of  ore  in  place  can  be  made,  "ore  expectant,"  in  the 
same  sense,  forms  the  lowest  and  least  accurate.  Very 
few,  if  any,  mine  owners  would  sell  a  property  for  the  net 
value  of  the  "developed"  and  "developing  ore,"  unless 
they  had  good  reason  to  believe  the  mine  to  be  practically 
bottomed;  hence  the  necessity  of  having  a  phrase  to 
cover  such  estimates,  as  well  as  uniform  and  thoroughly 
recognized  rules  to  govern  their  compilation. 

These  three  phrases,  "ore  developed,"  "ore  being  de- 
veloped" and  "ore  expectant"  should  cover,  in  metal- 
liferous mines,  all  estimates  a  mining  engineer  is  called 
upon  to  make.  I  now  put  them  forward  for  the  consid- 
eration of  the  profession,  not  as  the  best  possible  names 
and  classifications,  but  as  sound,  useful  and  definite 
phrases  to  govern  ordinary  estimates  of  mine  valuation. 
They  will  cover  any  condition  I  have  met  with  in  a  prac- 
tice extending  over  25  years  in  this  and  other  countries ; 
and  they  have  not  only  proved  useful  in  my  work,  but  are 
also  the  best  phrases  I  can  evolve  to  correctly  designate 
clear  and  concise  estimates. 

Every  report  on  a  mining  property  should  give  the  cost 
of  stores,  the  mining  and  milling  cost,  and  the  gross  and 
net  values  of  the  ores ;  therefore  it  should  not  be  a  neces- 
sary requirement  to  have  "ore  developed"  always  pay- 
ore.  The  gross  and  net  values  at  the  time  the  examina- 
tion is  made  will  fully  govern  the  then  value,  while  at 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

later  dates  the  improvements  in  mining-  and  milling  can 
usually  be  depended  on  to  increase  the  net  value,  often 
to  the  extent  of  showing  good  profits,  in  the  course  of  a 
few  years,  on  formerly  practically  valueless  ores.  The 
gross  value  is,  when  reported  in  ounces  and  percent- 
ages of  metals,  always  a  fixed  point  of  departure,  from 
which  the  net  value  can  at  all  future  times  be  deduced 
when  the  price  of  the  metals  and  cost  of  treatment  are 
taken  into  consideration. 

It  may  be  the  ambiguous  phrase  ''ore  in  sight"  is  so 
deeply  rooted  in  the  minds  of  mining  operators,  pro- 
moters and  investors,  that  it  cannot  well  be  replaced 
by  a  better  one.  I  hope  this  is  not  so ;  nevertheless, 
should  these  notes  provoke  a  sound  discussion  of  the 
whole  subject,  preparing  the  way  for  definite  classifica- 
tions of  ore  estimates,  with  clear  and  concise  rules  gov- 
erning each  class,  I  shall  feel  that  my  efforts  have  been 
to  some  purpose. 

In  conclusion,  permit  me  to  state  that  the  various 
methods  of  mine  sampling,  and  the  possible  errors  in 
estimating  tonnage  and  value,  are  excluded  from  the 
scope  of  this  letter.  I  simply  desire  to  present  for  the 
consideration  of  mining  engineers  the  classifications 
above  outlined,  and  which  I  summarize  as : 

Ore  Developed. — Absolutely  and  without  variation, 
ore  exposed  on  all  sides. 

Ore  Being  Developed.— First  class,  blocks  with  one 
side  hidden ;  second  class,  blocks  with  two  sides  hidden ; 
third  class,  blocks  with  three  sides  hidden. 

Ore  Expectant. — The  prospective  value  of  a  mine  be- 
yond or  below  the  last  visible  ore,  based  on  the  fullest 
possible  data  (clearly  set  forth)  from  the  mine  being 
examined,  and  from  the  characteristics  of  the  mining 
district. 


COST  PER  TON  AS  A  BASIS  OF  MINE 
VALUATION.* 

Bv  R.  Oilman  Brown. 

Hardly  second  in  importance  to  the  determination  of 
value  per  ton,  by  careful  and  systematic  sampling,  is  the 
determination  of  cost  per  ton  from  the  mine  accounts. 
The  object  of  sampling  has  been  attained  when  the  gross 
valuable  contents  of  the  ore  exposed  in  the  mine  have 
been  determined.  This,  for  the  sake  of  convenience  in 
subsequent  calculation,  is  usually  reduced  to  value  per 
ton.  From  this  gross  value  per  ton  must  be  deducted  the 
total  cost  per  ton  and  the  loss  per  ton  in  treatment,  the  re- 
mainder being  the  net  value  per  ton.  Obviously  an  error 
in  cost  per  ton  is  just  as  important  as  an  error  in  gross 
value  per  ton,  so  that  it  is  necessary  to  examine  with  as 
careful  scrutiny  the  method  of  obtaining  the  one  as  of 
the  other.  Several  pitfalls  have  been  pointed  out  recently 
in  the  columns  of  this  Journal  which  will  be  ignored  in 
this  present  article ;  or,  rather,  will  be  assumed  to  have 
been  satisfactorily  avoided,  the  desire  being  not  to  obscure 
another  factor  in  valuation  which  has  been  touched  upon 
but  lightly,  if  at  all. 

It  is  not  intended  in  the  present  article  to  go  into  the 
details  of  mine  accounting,  but  it  will  be  assumed  that  in 
some  way  or  other  the  engineer  has  obtained  certain  segre- 
gated totals  representing  the  expenditure  during  a  definite 
period  in  the  various  operations  connected  with  mining 
and  beneficiating  the  ore,  and  that  the  sum  total  of  these 
segregated  accounts  represents  the  total  cost  of  operations 
for  the  period  in  question. 

It  is  usual  to  divide  these  segregated  totals  by  the  tons 


^From  The  Engineering  and  Mining  Journal  of  August  29,  1903. 
—82— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

of  ore  treated  during  the  same  period,  and  to  call  the 
quotients  the  cost  per  ton  for  the  various  departments. 
In  a  certain  sense  the  sum  of  these  is  a  cost  per  ton,  though 
not  properly  to  be  considered  the  cost  per  ton,  as  the 
result  is  rather  a  hazy  approximation  representing,  per- 
haps, average  work  for  the  mine,  but  not  sufficiently  defi- 
nite to  be  used  with  confidence  in  the  delicate  work  of 
/aluing  ore  reserves. 

Let  us  assume  certain  natural  divisions  of  the  accounts 
into  stoping,  development,  reduction  and  general  expenses. 
All  the  segregations  can  be  easily  grouped  under  these 
headings ;  and  this  would  naturally  be  done  in  the  process 
of  arriving  at  the  cost,  in  the  manner  to  be  described.  In 
the  inexact  method  commonly  employed,  these  totals 
would  all  be  divided  by  the  same  tonnage — probably  by 
the  tonnage  reported  from  the  mill  or  reduction  works, 
whereas,  this  would  only  give  true  results  for  the  last 
two  items,  namely,  reduction  and  general  expense.  For 
stoping,  the  result  might  be  approximate  except  in  the 
case  of  a  mine  carrying  a  considerable  amount  of  ore  in  the 
stopes.  For  development,  it  could  only  give  the  true  value 
when  the  same  amount  of  ore  was  stoped  as  was  developed 
during  the  period.  If  development  fell  behind,  the  result 
])er  ton  would  be  too  little ;  if  it  exceeded  the  output,  it 
would  be  too  much. 

This  is  the  general  statement  of  the  matter,  but  it  can 
be  made  clearer  by  considering  a  concrete  case.  It  is 
proper  to  say  that  this,  though  based  on  practice,  has  been 
elaborated,  with  the  assumption  of  certain  values,  to  suit 
more  general  conditions. 

The  period  is  12  months. 

Tons  treated,  50,000. 

Total  return  per  ton,  $5.26. 

Total  loss  per  ton,  64c. 

Total  gross  value  per  ton  in  ore,  $5.90. 

Below  is  given  in  tabulated  form  the  costs  of  the  va- 
rious departments  and  the  costs  per  ton,  as  commonly 
calculated : 

-83— 


SAMPLING    AND    ESTIMATION    OF    ORE    l^if    A    MINE 

'  '■    Per  ton 

on  basis 

of  tons 

Total.  treated. 

Stoping $58,000  .  .  V-, ,     $1.16 

Development 50,000  '^     '.    i.oo 

Reduction   92,000  1.84 

General   expenses 22,000  .44 


Total    $222,000  $4-44 

The  following  tonnages  are  also  given : 

Tons. 

Ore  broken  in  the  stopes 10,000 

Ore  standing,  blocked  out,  ready  for  breaking .100,000 

Ore  reasonably  expected,  but  partly,   if  at  all,  developed. . .  50,000 


Total  160,000 

:These  are  the  quantities  assumed  to  be  found  by  the 
engineer  on  his  investigation  of  the  property,  and  the  re- 
coverable value  per  ton  will  be  taken  at  the  average  of  the 
past  year,  w^iich  we  will  assume  to  have  been  determined 
to  be  the  fact  by  careful  sampling.  The  most  natural  way 
<5f  calculating  the  value  of  these  reserves  would  be  as 
fellows : 

Total  recoverable  value  of  160,000  tons,  at  $5.26 $841,600 

Costs  on  j6o,ooo  tons,  at  $4.44 710,400 


•  '■  Apparent  profit  in  reserves $131,200 

This  is  manifestly  incorrect.  Stoping  and  develop- 
ment should  not  be  charged  against  ore  already  broken, 
'nor  should  -development  be  charged  to  that  standing 
blocked  out  and  ready  for  stoping.  The  estimate  should 
'read  as  follows,  assuitiing  the  figures  for  cost  per  ton 
to  be  correct:  ' 

10,000  tons  broken  in  stopes,  at  $2.28  per  ton  for  milling 
and  general   expense $22,800 

100,000  tons  developed,  ready  for  stoping,  at  $344  ($2.28 
+  $1-16) 344,000 

50,000  tons,  reasonably  expected,  but  not  developed,  at 
$444    222,000 


Total   costs $588,800 

160,000  tons,  recoverable  value  as  above 841,600 

Net   profits 252,800 

The  64use  of  the  great  difference  is  obviOtis.     It  should 
be  noted  that  in  the  accounts,  transportation  arid  handling 

— S4— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

costs  are  assumed  to  belong  in  general  expenses.  The 
above  figures  must  be  taken  as  thrown  in,  apart  from  the 
fundamental  matter  with  which  this  article  opened  and 
they  contain  errors,  as  was  indicated  in  our  opening  lines. 

Reduction  costs  and  general  expense  are  properly  re- 
duced to  the  per  ton  basis  by  dividing  by  50,000  tons,  but 
stoping  and  development  cannot  properly  be  so  reduced 
until  we  know  whether  50,000  properly  represents  the  tons 
stoped  and  the  tons  developed.  In  what  follows  we  are 
using  some  quantities  that  an  engineer  examining  the 
property  can  scarcely  have  come  at  directly,  and  indeed 
he  will  be  fortunate  if  he  has  them  at  all ;  but,  recognizing 
their  importance,  by  inquiry  and  estimate,  he  must  arrive 
at  some  sort  of  a  probable  value  for  them. 

We  have,  as  a  general  proposition,  that  the  actual 
amount  of  ore  broken  during  the  year  is  equal  to  the  ore 
on  hand  broken  at  the  end  of  the  year,  plus  the  ore  treated 
during  the  year,  and  minus  the  ore  on  hand  broken  at  the 
beginning  of  the  year.  In  the  same  way  the  ore  devel- 
oped during  the  year  is  equal  to  the  ore  blocked  out  at  the 
end  of  the  year,  plus  the  tons  of  ore  broken  during  the 
year,  and  minus  the  ore  standing  blocked  out  at  the  be- 
ginning of  the  year.  Applying  this  to  our  concrete  case, 
we  will  assume  the  following  values  for  tonnage  at  the 
beginning  of  the  year : 

Tons. 

Broken  in  the  stopes 20,000 

Standing  blocked  out 120,000 

Reasonably  expected 60,000 

Ten  thousand  tons  plus  50,000  minus  20,000,  equals 
40,000;  which  turns  out  to  be  the  actual  ore  stoped  during 
the  year,  so  that  the  true  cost  of  stoping  per  ton  is  58,000 
divided  by  40,000,  equals  $1.45  per  ton.  In  the  same  way 
100,000  plus  40,000  minus  120,000,  equals  20,000  tons,  the 
actual  ore  developed  during  the  year;  and  $50,000,  the 
total  spent  on  development,  divided  by  20,000  tons,  equals 
$2.50  per  ton,  the  true  cost  of  developing  a  ton  of  ore. 
Inasmuch  as  no  costs  are  credited  in  this  discussion  to  the 
ore  reasonably  expected,  we  need  not  carry  this  further, 

—85— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

though  the  same  reasoning  would  hold  in  regard  to  it. 
Collecting  our  true  cost  per  ton  we  have : 

Sloping   $1.45 

1  )evelopment   2.50 

Reduction  1.84 

General  expenses 44 

Giving  the  true  total  cost  per  ton  at $6.23 

This  shows  that  the  ore  at  this  mine  will  not  actually 
pay  expenses  for  handling  unless  greater  economies  can 
be  practiced;  whereas,  our  first  calculation  gave  a  profit 
per  ton  of  about  80  cents.  To  a  certain  extent,  then,  the 
matter  concerns  the  mine  manager  as  well  as  the  exam- 
ining engineer,  though  it  is  in  the  interest  of  the  latter  in 
particular  that  this  article  was  prepared. 

Proceeding  next  with  the  valuation  of  ore  reserves,  we 
reproduce  our  earlier  figures  with  the  corrected  costs 
introduced : 

10.000  tons,  broken,  at  $2.28  per  ton $22,800 

100,000  tons,  developed,  ready  for  breaking,  at  $373  ($2.28 

+  $1.45)  per  ton 373,ooo 

50,000  tons  ore,  reasonably  expected,  at  $6.23 311,500 

Total    $707,300 

Recoverable  value,  as  above 841 ,600 

Net  value  on  the  reserves 134,300 

In  order  to  lay  the  whole  matter  more  clearly  open  and 
even  at  the  risk  of  being  tedious,  it  is  worth  our  while  to 
make  another  assumption  regarding  tonnages  at  the  be- 
ginning of  the  period,  as  below : 

Tons. 

Broken  in  the  stopes None. 

Standing  ready  for  breaking 80,000 

The  tonnage  and  cost  per  ton  follow  in  summary : 

Tonnage  Cost 

for  year.  per  ton. 

Stoi-ing    60,000  97c. 

Development  80,000  62c. 

Reduction  and  general  expense,  as  be  fore.  50,000  $2.28 

Total    $3.87 

On  this  basis,  the  value  of  the  ore  reserves  is : 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

10,000  tons,  broken  in  stopes,  as  before $22,800 

100,000  tons,  ready  for  stoping,  at  $3.25    ($2.28  4"  97c.) 

per    ton 325,000 

50,000  tons  ore,  reasonably  to  be  expected,  at  $3.87 I93,500 

Total  charges  $541,300 

The  gross  recoverable  value,  as  before 841,600 

Profit  in  reserves  300,300 

The  diflferences  shown  clinch  the  statement  with  which 
this  paper  opened. 

As  a  summary  of  the  above  there  are  two  general  prin- 
ciples to  be  enunciated: 

(A).  Cost  per  ton  is  the  quotient  of  total  cost  divided 
by  the  actual  tonnage  resulting  from  that  expenditure. 

(B).  Net  value  per  ton  is  the  gross  recoverable  value 
per  ton,  less  cost  per  ton  for  those  natural  divisions  of  the 
work  still  to  be  performed  upon  the  different  classes  of 
ore. 

Although  the  above  principles  are  fundamental,  like  all 
general  statements,  they  may  easily  be  pressed  to  ex- 
tremes, and  it  is  for  the  engineer,  after  analysis  of  the 
situation,  to  decide  the  particular  extent  of  their  applica- 
tion ;  a  failure  to  recognize  them  at  all,  on  the  other  hand, 
may  invalidate  the  conclusions  drawn  from  otherwise  re- 
liable work. 


MINE  VALUATION.* 

By  J.  Parke  Channing. 

May  it  not  be  possible  to  have  a  mine  with  too  much 
"ore  in  sight"?  I  ask  this  question  because  the  gen- 
eral tenor  of  the  discussion  on  mine  valuation  has  made 
"ore  in  sight"  the  crucial  test  of  the  value  of  a  property. 

If  based  on  this  test,  many  of  our  good  iron  mines 
would  make  a  sorry  showing;  and  yet  their  capacity  for 
production  is  well  known,  and  year  after  year  they  come 
up  to  expectation.^  If  one  has  a  fairly  regular  deposit,  is 
an}i:hing  to  be  gained  by  rushing  the  openings  so  that  lo 
or  20  years'  supply  of  ore  is  opened  up  ?  Will  not  the  in- 
terest charges  on  the  cost  of  this  development,  the  expense 
of  re-timbering  the  openings  and  the  pumping  charges  be 
a  heavy  drain  on  the  income  ? 

In  Butte,  to-day,  litigation  has  forced  development  to 
such  excess  that  it  is  estimated  that  certain  drifts  will  have 
to  be  re-timbered  four  times  before  the  ore  is  stoped. 
Several  of  the  prominent  Lake  Superior  copper  mines 
have  ground  opened  up  many  years  in  advance  with  but 
questionable  benefit,  as  the  drifts  driven  every  loo  ft.  do 
not  show  the  full  width  of  the  lode  and  the  rock  from 
these  openings  has  not  been  stamped  by  itself,  so  as  to 
show  its  yield.  Cutting  out  stopes  every  300  ft.  would  at 
least  have  been  cheaper  and  have  given  more  idea  of  the 
yield  to  be  expected. 

The  president  of  a  prominent  western  railway  boasted 
that  he  had  just  added  to  the  company's  coal-land  hold- 
ings a  90  years'  supply.  A  little  figuring  showed  him 
that  the  interest  on  his  investment  would  make  his  coal 
90  years  hence  pretty  expensive. 

A  few  years  ago  Prof.  Robert  Peele,  Dr.  John  H.  Banks 
and  myself  were  required  to  place  a  valuation  on  four 

•From  The  Engineering  and  Mining  Journal  of  September  12,  1903. 

—8a— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

pyrite  mines  for  a  proposed  consolidation.  They  were  in 
various  stages  of  development,  and  one  of  them  had  more 
ore  in  sight  than  the  combined  tonnage  of  the  other  three. 
This  large  tonnage  left  in  the  mine  consisted  of  ore  which 
in  the  earlier  days  would  have  produced  fines  then  of  little 
value.  At  the  time  of  the  examination  the  development 
of  mechanical  roasting  furnaces  had  placed  fines  on  a  par 
with  lump,  and  this  ore  was  now  available  for  extraction. 
The  question  arose,  What  value  was  to  be  given  to  this 
ore  in  sight  ?     I  quote  from  our  report : 

"We  have  taken  the  ground  that  in  mines  of  this  char- 
acter the  deposits  are  so  regular  that  the  better  test  of 
their  relative  values  is  to  be  found  in  their  capacity  for 
production  per  lOO  ft.  of  depth,  rather  than  in  the  number 
of  tons  of  ore  available  for  immediate  shipment;  that  is, 
*in  sight.'  Taking,  as  we  do,  this  view,  we  desire  to  mini- 
mize the  effect  produced  on  the  value  of  each  mine  by  the 
varying  amounts  of  ore  in  sight,  believing,  for  example, 
that  by  sinking  B  mine  and  C  mine  each  about  200  ft.,  and 
running  the  necessary  drifts,  we  would  have  in  each  of 
these  mines  an  amount  of  ore  in  sight  equal  to  that  at  A 
mine.  This  result  would  be  accomplished  by  the  expen- 
diture of  a  comparatively  small  amount  of  money  in  each 
case,  which  would  be  charged  to  development." 

Following  this  line  of  reasoning,  and  attempting  per- 
haps the  rather  difficult  task  of  "giving  mathematical  ex- 
actness to  phenomena  governed  by  the  complex  laws  of 
nature,"  we  evolved  the  following  formula : 
V  =  5  a,r  -}-  by  -\-  c,  in  which — 
V  =  Value  of  the  mine. 
'       a  =:  Net  profit  per  ton  of  ore  mined. 
x  =  Annual  estimated  output  in  tons. 
b  =  Cost  of  development  per  ton  of  ore  opened  up. 
y  =  Tonnage  of  ore  in  sight  above  the  bottom  level. 
c  =  Value  of  the  equipment  and  stocks  on  hand. 

In  the  above  formula  it  will  be  seen  that  the  practical 
valuation  of  the  mine  was  assumed  at  five  times  the  an- 
nual profit.  The  figures  substituted  in  the  above  formula 
for  the  four  mines  were  as  follows  :  .   ! 

-89- 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

X  y 

a           Tons.           b  Tons.                c-  V 

A $0.84        96,500  $o.o(32  293,000  $97,100  $520,566 

B 0.94      130.500  0.058  88.800  115,400  686,900 

C 1. 15        79.500  o.ioo  117,200        67,650  536,495 

D 0.91         15,000  0.150  63,000        35,100  112,800 

Some  three  years  later  I  had  almost  the  same  problem 
in  a  group  of  copper-silver-gold  properties  in  a  western 
State  which  it  was  proposed  to  consolidate,  as  the  various 
ores,  with  the  addition  of  a  little  limestone,  would  make  a 
most  economical  smelting  mixture.  In  this  problem  the 
copper  mines  E  and  H  were  well  opened  up,  while  the  two 
silicious  gold  and  silver  mines  F  and  G  had  not  the  open- 
ings and  development  that  proper  mining  would  demand. 
The  bottoms  looked  fairly  well  and  the  past  records  were 
good,  but  no  such  assurance  of  "future  performance"  was 
to  be  counted  on  as  in  the  regular  l^yrite  mines  previously 
described. 

From   the   data  available   the   following   figures   were 

compiled :  Owner's 

ay  -\-  c  ax  price. 

E $800,000  $284,375  $6,000,000 

F 128,000  1 12,000  250,000 

G 866,000  630,000  3,200,000 

H 600,000  175,000  750,000 

$2,394,000  $1,201,375  $10,200,000 

In  the  above  ay  -f  c  represented  the  profit  in  sight,  the 
tonnage  in  sight  y  being  multiplied  by  the  estimated  profit 
per  ton  a,  instead  of  by  b,  as  in  the  pyrite  mines. 

The  figures  ax  were  only  tentative  in  this  case,  and 
would  have  to  be  used  as  a  guide  only. 

The  scheme  of  consolidation  was  never  carried  out  on 
account  of  the  high  valuation  placed  upon  their  properties 
by  the  owners  of  E  and  G.  The  owners  of  E  wanted  60 
per  cent  of  the  consolidated  stock,  while  they  had  but  33 
per  cent  of  the  total  value  in  sight  and  were  capable  of 
contributing  only  25  per  cent  of  the  earnings.  The  own- 
ers of  G  were  not  unreasonable  in  their  demands,  were  it 
not  for  the  fact  that  their  future  was  doubtful  and  with 
only  a  little  over  one  year's  ore  in  sight  they  were  demand- 
ing a  five  years'  purchase. 

Had  the  decision  been  in  my  hands  I  would  have  con- 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

solidated  on  the  following  basis :  E,  $2,000,000 ;  F,  $250,- 
000 ;  G,  $2,000,000 ;  H,  $750,000 ;  total,  $5,000,000. 

I  give  these  two  cases,  the  first  showing  the  extreme 
limit  in  which  ''profit  in  sight"  is  almost  negligible.  The 
other  limit  in  which  "profit  in  sight"  is  a  sine  qtia  non  has 
been  gone  into  most  thoroughly  by  your  previous  con- 
tributors. My  second  case  is  a  mean  between  the  two; 
and  in  estimating  the  value  of  a  property  either  as  an 
arbiter  or  for  a  purchaser  the  engineer  must  integrate  his 
equation  between  the  two  limits.  Buying  a  mine  on  the 
basis  of  "profit  in  sight"  is  much  like  dealing  in  pork  or 
dry  goods,  and  needs  not  the  services  of  a  trained  engineer 
as  much  as  it  does  careful,  plodding  samplers  and  as- 
sayers. 

What  I  wish  to  bring  home  to  the  engineer  examining 
properties  for  an  investor  is  the  point  that  his  client's 
greater  profits  are  not  to  be  made  from  the  purchase  of 
those  properties  where  the  "profit  in  sight"  is  so  great,  but 
from  those  which  the  engineer's  judgment  tells  him  will 
improve  wdth  development.  As  the  late  Marcus  Daly  said 
to  one  of  his  men  who  had  spent  two  years  in  fruitless 
search  for  a  mine,  "Why  the  devil  don't  you  get  reckless 
once  in  a  while  and  spend  forty  or  fifty  thousand  dollars  ?" 


— <>t— 


SAMPLING   AND    ESTIMATION    OF    ORE   IN    A   MINE 


^92 — 


DISCUSSION. 

ORE  DEVELOPED. 

The  Editor: 

Sir. — it  is  with  some  diffidence  that  I  venture  to  ques- 
tion the  advantage  of  Mr.  PhiHp  Argall's  proposed 
classification  of  ore-tonnage  estimates  as  used  in  mine 
reports.  I  am,  of  course,  ready  to  agree  that  "ore  in 
sight"  is  always  an  unwise  and,  frequently,  a  misleading 
expression,  as  appHed  to  ore-ground  in  place ;  the  phrase 
is  sufficiently  discredited  by  this  time  for  its  use  to  be 
discontinued  by  general  consent. 

But  may  not  any  other  definition  prove  misleading  to 
a  less  extent  ?  Take,  for  instance,  Mr.  Argall's  "ore  de- 
veloped"— meaning,  in  all  cases,  "ore  exposed  on  all 
sides."  Most  of  us  have  known,  many  of  us  have  ex- 
perienced, cases  where  blocks  of  ore  so  exposed  on  four 
sides  have  been  found  to  enclose  a  large  barren  patch 
in  the  center.  However  rigid  a  rule  we  may  adopt,  we 
cannot  arrive  at  certainty. 

The  amount  of  weight  to  be  attached  to  exposure  on 
four  sides,  on  three,  on  two,  or  one  one  only,  varies  with 
the  conditions  of  each  particular  case.  Fig.  i,  for  in- 
stance, shows  a  section  of  vein-workings  from  which, 
with  other  data,  estimates  were  made  by  my  brother, 
the  late  Arthur  L.  Collins,  under  the  three  headings  of 
"positive  ore,"  "probable  ore,"  and  "possible  ore." 
Who,  after  examining  the  section,  and  bearing  in  mind 
the  phenomenal  lateral  extension  of  the  ore-body  shown, 
can  doubt  that  far  more  reliance  can  be  placed  on  blocks 
marked  in  the  section  as  "positive  ore,"  even  though 
some  of  them  are  exposed  on  three  sides  only,  than 
on  many  blocks  opened  up  on  four  sides  in  veins  where 
the  ore-bodies  are  of  the  bunchy  and  erratic  type  which 
we  must  recognize  to  be,  after  all,  by  far  the  most  fre- 


SAMri.IXC.    AND    ESTIMATION    OF    ORE    IN    A    MINE 

quently  encountered?  Even  the  "possible  ore"  of  Fig. 
I — ground  where  the  ore  is  proved  to  exist  on  one  side 
only — can  be  depended  on  to  a  far  greater  extent  than 
usual. 

My  contention  is  that  the  amount  of  evidence  to  be  re- 
quired when  making  estimates  of  ore-tonnage,  and  the 
number  and  nature  of  classes  into  which  estimates 
should  be  divided,  must  depend  on  our  general  conclu- 
sions as  to  the  nature  and  permanence  of  the  ore- 
bodies  in  the  mines  under  consideration,  the  distance 
between  the  workings,  etc.  I  do  not  think  that  any 
rules  can  be  made  which  will  lessen  the  necessity  for  -de- 
pendence on  the  examining  engineer's  individual  judg- 
ment; and  I  distrust  all  cast-iron  classifications,  which 
do  not  allow  for  the  infinite  complexity  of  natural  con- 
ditions. The  only  useful  general  rule  is  that  no  estimate 
of  tonnage  should  be  made  unless  accompanied  by 
sketches  indicating  the  basis  on  which  it  rests. 

I  understand  Mr.  Argall  to  suggest  that,  as  it  is  fre- 
quently not  advisable  in  the  ordinary  course  of  mining 
to  block  out  ore  so  as  to  come  under  his  definition  of 
"ore  developed,"  parties  desirous  of  selling  should  go  to 
the  additional  expense  of  doing  so.  While,  however, 
I  admit  that  the  valuation  of  mines  for  sale-purchase  is  a 
very  important  duty  of  the  mining  engineer,  I  cannot  see 
that  methods  of  mining  should  be  modified  to  suit  it.  A 
mine  primarily  exists  for  the  purpose  of  being  worked  to 
the  greatest  economical  advantage,  and  an  engineer  must 
expect  to  make  his  estimates  conform  to  the  particular 
case.  There  are  many  instances  where  the  additional 
openings  which  would  become  necessary  to  bring  proved 
ore-ground  within  the  limits  of  Mr.  Argall's  definition  of 
'ore  developed"  would  increase  the  cost  of  exploitation 
to  such  a  figure  that  the  profit-margin  would  be  seriously 
curtailed. 

Take,  for  instance,  the  ore-body  shown  in  Fig.  2 — a 
case  which  is  typical  of  perhaps  the  largest  number  of 
ore-bodies  in  fissure-veins.  Such  ore-bodies  are  usually 
most  economically  developed  by  a  single  raise  through 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

the  ore,  as  shown.  According  to  Mr.  Argall's  classifica- 
tion there  is  no  "ore  developed."  Yet,  certainly  the  ore 
is  there,  more  or  less;  it  is  certainly  developed,  so  far 
as  it  is  economically  required  to  be.  I  have  indicated 
by  shading  the  basis  which  I  should  have  taken  for  the 
purpose  of  making  an  estimate  of  ore  developed.  Such 
ore  is  by  no  means  certain,  nor  is  any  other ;  but  I  should 
use  such  words  in  referring  to  it  as  would  clearly  signify 
with  what  degree  of  confidence  I  regarded  it.     Actual 


777T77T7T77777\  Limits  of  pay  ore   as 
'f'/////,'////,'/^,   ultimately  worked  out. 

Fig  2. 
Ore  Developed,  as  Estimated  from  Workings  Shown. 

certainties  as  to  ore  are  reached  only  after  it  has  been 
crushed  and  sampled ;  and  I  might  almost  add,  paid  for. 

It  would  be  a  mistake  to  sacrifice  the  two  essentials, 
clearness  and  accuracy,  for  the  sake  of  getting  concise 
and  well-sounding  phrases;  and  equally  so  to  attempt 
to  classify  all  possible  estimates  of  ore  under  prejudged 
headings.  Nature  does  not  always  plan  ore-occurrences 
to  suit  our  systems  of  classification. 

While  it  hardly  comes  within  the  limits  of  the  same 


-95- 


SAMPLING    AND    KSTIMATIOX    OF    ORE    IN    A    MINE 

discussion,  I  would  say  a  word  in  favor  of  the  prospect- 
ing pick  for  purposes  of  mine  sampling,  notwithstand- 
ing the  authority  of  Mr.  T.  A.  Rickard.  I  have  found 
that  by  the  alternate  use  of  the  pick  and  hammer  ends, 
very  fair  samples  can  be  taken  in  all  but  exceptionally 
tight  ground,  where  the  single  jack  and  moil  become 
necessary.  It  has  the  additional  advantage  that  an  en- 
gineer can  take  his  samples  personally,  and  that  the  time 
and  labor  of  sampling  are  greatly  lessened ;  points  of 
which  Mr.  Rickard  would  be  the  first  to  admit  the  sig- 
nificance. It  certainly,  however,  requires  more  indi- 
vidual judgment  on  the  part  of  the  sampler. 

I  remember  a  case  where  I  took  a  large  number  of 
samples  alone,  some  time  after  sampling  by  a  very 
prominent  firm,  the  evidence  afforded  by  their  grooves 
still  standing  to  show  that  the  work  was  properly  done. 
In  this  case  the  subsequent  working  of  the  mine  proved 
that  my  average  was  more  reliable ;  by  no  means  because 
it  was  done  in  a  more  honest  or  painstaking  manner. 

George  E.  Collixs. 
Denver,  Colo.,  Feb.  24,  1903. 


The  Editor: 

vSir. — The  letter  of  Mr.  George  E.  Collins,  in  the  issue 
of  the  Journal  for  March  14,  1903,  continuing  the  dis- 
cussion in  regard  to  sampling  and  estimates  of  ore,  in- 
duces me  to  offer  a  few  suggestions  along  the  same  line 
of  criticism.  Mr.  Collins  says :  'The  amount  of  weight 
to  be  attached  to  exposure  on  four  sides,  on  three,  on 
two,  or  on  one  only,  varies  with  the  conditions  of  each 
particular  case.''  And  further  on,  "And  I  distrust  all 
cast-iron  classifications,  which  do  not  allow  for  the 
infinite  complexity  of  natural  conditions." 

However  much  ability  the  man  to  whom  the  examina- 
tion of  a  mine  is  entrusted  may  have  to  sample  the  avail- 
able ore  geometrically  blocked  out,  he  is  unworthy  the 
confidence  reposed  in  him  if  he  is  unfamiliar  with,  or 
neglects   to   take   cognizance   of,   the   principles   which 

—96— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

govern  the  deposition  of  ores.  A  knowledge  of  these 
geological  principles  implies  not  only  mineralogical 
training,  and  a  knowledge  of  the  broad  general  rules  hv 
which  ore-deposits,  through  the  accumulation  of  ex- 
perience, have  been  classified,  but  it  includes  a  consid- 
eration of  the  nature  of  the  enclosing  and  adjacent  rocks, 
and,  what  is  more  important,  a  recognition  of  the  phe- 
nomena of  ore-shoots. 

The  much-abused  term,  ore-shoot,  or  shoot  of  ore,  as 
applied  to  mineral  veins,  may  be  defined  as  that  portion 
of  a  metalliferous  deposit  within  whose  boundaries  the 
mineral  mass  is  commercially  valuable.  This  definition 
purposely  omits  specifying  the  shape  of  an  ore-shoot, 
and  any  reference  to  the  reasons  for  its  existence.  It  is 
evident  that  the  form  and  size  of  the  ore-shoots  in  a 
given  mine  must  depend,  not  on  any  scientific  law,  but, 
for  the  most  part,  upon  the  actual  working  expenses. 
In  a  mine  which  has  been  worked  for  a  period  of  years, 
th'e  worked-out  portions  of  the  vein  usually  afTord,  by 
their  form  and  dimensions,  a  fairly  reliable  index  of  th-e 
size  and  shape  of  the  ore-shoots  for  that  mine,  but  for 
that  mine  alone. 

The  mining  engineer  who  examines  and  reports  on  a 
mine  in  which  the  ore-shoots  have  th'us  been  defined  by 
local  experience  may  commit  an  act  of  injustice  to  the 
intending  purchasers,  to  the  vendors  and  to  himself, 
if  he  neglects  to  moderate  his  calculations,  based  upon 
the  actual  sampling  of  ore  blocked  out,  by  a  proper 
consideration  of  the  system  of  ore-shoots  as  developed 
by  previous  workings. 

In  a  mine  whose  workings  are  on  a  fissure-vein,  ex- 
perience has  shown  that  the  ore-shoots  lying  in  the 
plane  of  the  vein  have  frequently  a  considerable  amount 
of  regularity,  as  regards  size  and  distribution.  This  is 
especially  the  case  in  veins  whose  principal  gangue  is 
quartz,  and  whose  values  consist  of  any  or  all  of  the 
minerals,  gold,  silver,  lead  and  copper.  The  engineer 
sh'ould  consider  it  a  part  of  his  duty  to  ascertain  if 
the  characteristics  of  the  ore-shoots  in  the  vein  he  is 

—97— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN,  A    MINE 

examining  are  peculiar  to  that  particular  mine,  or  are 
duplicated  in  other  mines  of  the  same  district.  The 
gathering  of  such  data  is  frequently  rendered  impossible 
by  local  jealousy  or  other  motives.  The  data  are  usually 
obtainable,  however,  and  sometimes  detailed  pubHshed 
maps  of  the  district  in  question  exist.  In  most  large 
mining  districts  of  the  world,  both  surface  and  under- 
ground data  have  been  published  and  are  available  for 
those  who  care  to  use  them.  If  the  vein  under  examina- 
tion is  one  of  a  series,  upon  which  operations  in  different 
-mines  have  been  conducted  for  a  long  period,  and  which 
have  been  shown  to  present  a  uniformity  or  similarity  of 
conditions,  the  engineer  has  a  largely  increased  supply 
of  material  at  hand  from  which  he  may  draw  conclu- 
sions. 

To  take  another  example  of  ore-deposits,  if  the  mine 
in  question  is  operated  on  a  silver-lead  bedded  lime- 
stone deposit,  of  the  kind  sometimes  referred  to  as  a 
"'contact,"  the  form  of  one  ore-shoot  may  afford  no  clue 
as  to  the  form  of  the  next  one  to  be  encountered.  From 
the  very  origin  of  such'  a  deposit,  it  may  generally  be 
assumed  that  the  occurrence  of  the  ore  will  be  subject  to 
less  definite  laws  than  is  the  case  in  fissure-veins.  It 
is  occasionally  found,  however,  that  a  definite  local  dis- 
position of  the  ore  in  shoots  in  a  bedded  silver-lead 
or  zinc  deposit  occurs.  The  engineer,  in  examining  a 
mine  in  which  such  an  occurrence  has  been  proved  by 
the  work  already  done,  will  naturally  be  at  fault  if  he 
takes  no  account  of  the  history  of  the  property  and  its 
previous  underground  development.  He  will,  however, 
be  likely  to  exercise  unusual  caution  in  estimating  the 
three  classes  of  "positive,"  "probable"  and  "possible" 
ore,  when  dealing  with  a  class  of  deposits  which'  are 
genetically  connected  with  the  chemically  induced  re- 
placement of  limestone  by  metallic  sulphides. 

In  the  accompanying  diagram  (plan)  the  ore-body  rep- 
resented is  an  occurrence  in  a  limestone  bed,  say,  7  ft. 
thick.  The  bed,  itself  lying  flat,  is  heavily  mineralized. 
The  carbonate  of  lime  has  been  replaced  by  metallic  sul- 

—98— 


SAMPLINc;    AND    ESTIMATION    OF    ORE    IN    A    MINE 

phides  carrying  values.  The  enclosing  beds,  namely, 
those  lying  above  and  below,  are  barren,  and  the  min- 
eralized bed  itself  contains  barren  patches,  sporadically 
distributed.  Assume  that  the  ore-body  thus  defined,  a 
portion  of  which  is  here  represented,  had  dimensions  as 
follows :  Seven  feet  thick,  that  is  the  thickness  of  the 
limestone  stratum,   150  ft.  long  and  90  ft.  wide.     The 


Plan  of  a  Part  of  the  Workings  of  a  Silver-Lead 
Mine  in  Horizontally  Bedded  Limestone. 


four  tunnels  represented  are  all  run  in  this  ore-body, 
and  by  their  intersections  enclose  a  portion  of  it,  A  B 
C  D.  In  other  words,  a  block  of  ore  is  exposed  on  four 
sides,  7  ft.  thick,  45  ft.  wide,  and  70  ft.  long.  If  one 
should  apply  an  arbitrary  rule  of  sampling  in  the  ex- 
amination of  the  mine  in  which  this  case  occurs,  the  ore 
in  question  must  be  classed  under  the  head  of  ''positive." 
While  not  questioning  the  correctness  of  his  classifica- 
tion, I  have  endeavored  to  show,  by  the  diagram,  the 
error  to  which  the  examiner  would  be  liable,  should  he 
estimate  the  block  represented  by  A  B  C  D  as  ore.  On 
the  other  hand,  if  the  examiner  takes  into  consideration 


SAMPLIXC;    AND    ESTIMATION    OF    ORE    IX,  A    MINE 

the  geological  principles  on  which  the  class  of  limestone 
silver-lead  deposits  depends,  he  will  probably  allow  in 
his  calculations  for  the  possibility  of  the  presence  of  the 
patch  or  horse  of  barren  limestone  L.  At  any  rate, 
he  will  be  most  assiduous  in  his  efiforts  to  find  out  if 
patches  of  normal  limestone,  unaffected  by  the  ore-bear- 
ing solutions,  have  been  encountered  in  previous  work- 
ings of  the  mine,  or  in  the  surrounding  mines  of  similar 
character. 

The  term  ore-sh'oot  may  be  said  to  be  a  development 
of  the  experience  of  that  invaluable  member  of  the  min- 
ing profession,  the  mine  foreman.  His  every-day  con- 
tact wuth  the  ore,  and  with  the  profit  and  loss  conditions 
of  handling  it,  makes  him  keen  to  recognize  its  slightest 
variations,  so  far  as  can  be  judged  from  its  appearance. 
The  very  fact,  however,  that  the  term  "shoot  of  ore" 
has  primarily  a  commercial  and  not  a  scientific  defini- 
tion, has  resulted  in  an  exceedingly  loose  application  of 
it.  'Would  it  not  be  an  advisable  point  for  discussion 
as  to  whether  the  word  "shoot"  should  not  be  more 
closely  limited  in  its  uses  ? 

A  fissure  may  be  primarily  of  varying  width,  especially 
when  the  enclosing  walls  are  slates  or  metamorphic 
schists,  and  the  vein  is  conformable  in  strike  with  their 
schistosity.  The  vein-filling,  or  ore,  with  which  the 
fissure  has  been  filled,  may,  when  drifted  on,  present  at 
one  station  a  width  of  6  ins.,  and  at  another,  perhaps  lOO 
ft.  farther  along,  of  6  ft.  Assume  that  the  ore  carries  0.5 
oz.  in  gold.  There  may  be  a  recurrence  of  the  wide  and 
narrow  portions  of  the  fissure,  giving  a  succession  of 
lenses  of  ore.  Naturally  the  drift  must  be  carried  wide 
enough  for  the  operations  as  long  as  the  vein  is  followed. 
The  vein  will  be  stoped  only  in  those  portions  where  the 
vein-filling  widens  to  such'  an  extent  as  to  warrant  its 
being  extracted  and  milled,  or,  in  other  words,  where  it 
may  be  defined  as  pay-ore.  In  those  parts  of  the  vein 
where  the  vein  material  occupies  only  6  ins.,  or  possibly 
even  a  narrower  space,  no  more  ground  will  be  broken 
than  necessary,  and  the  foreman  will  define  the  inter- 


SAMPLIXCr    AND    ESTIAIATION    OF    ORE    IN    A    MINE 

vening  narrow  space  along  the  drift,  between  any  two 
lenses  of  ore,  as  barren  ground.  His  ''shoot"  will  be 
limited  in  its  length  along  the  drift  by  the  imaginary 
planes  between  which  the  two  walls  of  the  lenticular 
opening  in  the  vein  contain  sufficient  ore  to  be  profit- 
ably extracted.  All  inside  the  wide  part  of  the  vein  will 
be  the  "shoot,"  and  all  outside  will  be  reckoned,  for  the 
time  being  at  least,  as  ''barren  ground." 

On  the  other  hand,  take  the  example  of  a  fissure- vein, 
with  straight  parallel  walls,  and  with  a  width  of  3  ft. 
The  filling  is  quartz-gangue  containing  sulphides,  and 
the  width  does  not  vary  for  long  distances.  In  certain 
portions  the  operations  have  shown  that  high  values 
in  gold  and  silver  occur  in  the  vein-matter,  while  other 
portions,  indistinguishable  by  their  physical  character- 
istics from  the  ore,  are  practically  waste-rock.  I  necl 
not  recall  to  miners,  and  to  those  familiar  with  mines 
generally,  how  frequently  such  alternations  of  rich  and 
barren  vein-matter  occur. 

The  last  case  cited  is,  according  to  the  present  system 
of  nomenclature,  as  much  an  occurrence  of  ore  in  shoots 
as  is  the  instance  of  the  vein  of  varying  width  in  slate 
country,  where  the  distribution  of  the  shoots  depended 
on  the  form  of  the  original  fissure.  In  the  one  instance 
the  shoot  resulted  from  physical  causes;  in  the  other 
the  distribution  of  the  shoots  was  due  to  chemical  dif- 
ferentiation in  the  solutions  from  which  it  was  deposited. 
Explanations  of  the  selective  activity  exhibited  by  these 
solutions,  resulting  in  the  formation  of  shoots  of  ore, 
are,  up  to  the  present,  largely  speculative.  In  addition 
to  the  above  two  classes  of  ore-shoots  must  be  consid- 
ered those  occurring  in  connection  with  fissure-veins 
in  bedded  deposits.  In  these  the  chemical  action  result- 
ing in  their  formation  has  been  in  some  cases  controlled 
by  physical  causes.  A  consideration  of  them  is  hardly 
pertinent  to  the  point  I  have  raised  regarding  shoots 
in  fissure-veins. 

I  can  hardly  allow  to  pass  one  point  made  by  Mr.  Col- 
lins in  his  valuable  letter.     In  Fis:.  2,  of  the  illustra- 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

tions  accompanying  his  article,  he  shows  "a  case  which 
is  typical  of  perhaps  the  largest  number  of  ore-bodies 
in  fissure-veins."  The  shape  of  the  ore-body  figured  by 
Mr.  Collins,  I  am  inclined  to  think,  is  of  a  rather  excep- 
tional kind.  From  the  context  it  is  to  be  inferred  that 
Fig.  2,  of  Mr.  Collins,  represents  a  shoot  of  ore  in  a 
vein.  Shoots  of  ore  in  fissure-veins,  as  they  have  been 
defined  above,  my  experience  has  induced  me  to  believe, 
have  in  the  majority  of  cases  a  lenticular  form,  whatever 
their  origin.  A  definite  pitch  in  the  plane  of  the  vein  is 
generally  characteristic  of  them.  The  pitch  may  make 
any  angle  with  a  horizontal  line  representing  the  strike 
of  the  vein.  The  longest  axis  of  the  shoot  may  form  a 
right  angle  with  this  horizontal  line,  or  again  the  longest 
axis  of  the  shoot  may  coincide  with  the  horizontal,  con- 
stituting the  "course"  of  the  Cornish  miner.  The  main 
point  I  wish  to  make  is  that,  in  the  majority  of  cases 
hitherto  noted,  it  has  been  found  that  the  longest  axis 
of  an  ore-shoot  is  a  straight,  or  nearly  straight,  line,  and 
not  a  curved  line  in  the  sense  figured  by  Mr,  Collins. 

My  contention  is  not  that  ore-shoots  of  very  irregular 
form  do  not  occur,  and  the  above  criticisms  and  remarks 
are  offered  mainly  in  the  hbpe  that  those  who  have  ob- 
served the  phenomena  extensively  will  record  the  results 
of  experience.  The  literature  of  ore-shoots  is  not  ample, 
and  no  one  will  deny  that  the  subject  possesses  much 
practical  interest  and  importance  in  its  relation  to  the 
mining  industry. 


C.  W.   PURINGTON. 


Denver,  Colo.,  March  24,  1903. 


The  Editor: 

Sir. — This  excellent  series  of  articles  is  without  ques- 
tion the  best  presentation  of  the  difficult  and  important 
subject  of  mine  sampling  that  has  yet  appeared ;  neverthe- 
less, there  are,  I  believe,  a  few  points  that  have  not  been 
treated  as  fully  as  their  importance  demands,  and  some 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

others  where  there  is  at  least  room  for  discussion  as  to 
the  better  practice.  As  Mr.  Rickard  says,  **No  two  men 
set  to  work  in  exactly  the  same  way."  Each  person  nat- 
urally believes  his  own  system  the  best  or  he  would 
not  follow  it.  Consequently,  I  propose  briefly  describing 
the  methods  that  have  in  my  experience  given  the  best  re- 
sults in  such  cases. 

Size  of  Sample. — The  engineer  having  decided  the 
proper  distance  between  the  points  of  sampling,  it  next 
becomes  necessary  to  determine  the  safe  weight  of  the 
original  sample,  taking  fully  into  account  the  nature  of 
the  ground  and  its  breaking  qualities.  The  sample  is  then 
estimated  at  so  many  pounds  per  foot  in  width  of  the  vein. 
In  gold  and  silver  mines  from  2  to  lo  lb.  should  cover  the 
limits  in  ordinary  cases,  and  an  average  of,  say,  5  lb.  per 
foot  will  usually  be  found  quite  safe.  Thus  a  vein  10  ft. 
wide  would  give  a  50-lb.  sample.  This  factor  determined 
for  the  particular  vein  under  consideration,  the  samplers 
should  be  instructed  as  to  the  width  and  depths  of  the 
sample  cuts  required,  and,  as  a  check  on  the  accuracy 
of  their  work,  the  samples  should  be  weighed,  and  those 
exceeding  10  per  cent,  variation  from  the  adopted  stan- 
dard rejected.  The  irregular  weight  of  samples  is  a 
fruitful  source  of  error  in  mine  examination  because  the 
excess  weight  invariably  comes  from  the  softer  and 
usually  richer  portion  of  the  vein,  or  else  it  comes  from 
loose  or  shattered  rock  containing,  in  its  fissures,  con- 
centrated vein  material  not  rightly  belonging  to  a  true 
sample.  Large  samples  are  usually  a  mistake  in  the  sys- 
tematic sampling  of  a  gold  or  silver  mine,  and  so  we  rely 
on  "The  smallest  sample  consistent  with  securing  a  true 
average  of  the  lode  at  each  place  sampled." 

Marking  the  Position  of  the  Samples  — Where  arbitrary 
distances  are  chosen  for  the  points  of  sampling,  the  posi- 
tion of  each  cut  should  be  marked  off  in  advance  and  tied 
to  cross-cuts,  survey  stations,  and  other  prominent  marks, 
so  as  to  check  up  with  the  general  plan  of  the  mine.  The 
position  of  each  cut  is  best  marked  with  a  number,  pre- 
ferably the  sample  number,  painted  or  chalked  on  the  rock 

—103— 


SAMPLIN'G    AND    ESTIMATION    OF    ORE    IN    A    MINE 

at  pne  side  of  the  workings ;  this  facihtates  re-sampHng', 
or  the  examination  of  the  cut,  as  one  can  walk  directly  to 
the  point.  It  is  a  great  advantage  to  the  engineer  to  be 
able  to  examine  the  sample  cuts  while  they  are  yet  fresh, 
and,  having  with  him  the  assay  value  of  the  ore,  he  will 
then  quickly  see  from  the  nature  of  the  mineral  exposure 
those  places  that  appear  abnormally  high  or  low  and  order 
the  re-sampling  accordingly. 

Examining  the  Places  to  be  Sampled. — After  the  posi- 
tion of  the  proposed  sample  cuts  is  established  as  above 
explained,  the  vein  at  the  places  marked  is  carefully  ex- 
amined, brushed,  or  washed  clear  from  accumulated  dust, 


/vy*- 


mud  or  dope,  and  irregular  projections  knocked  off,  so 
that  the  cut  will  be  practically  at  right  angles  to  the  dip 
of  the  vein,  or,  at  least,  a  straight  line  across  the  ore  to  be 
sampled. 

In  Fig.  I  the  hard  portions — ^A,  B — should  be  removed 
before  sampling. 

Fig.  2  shows  at  once  one  of  the  most  difficult  samples 
to  take,  as  well  as  the  most  fruitful  source  of  error  in  mine 
sampling.  I  have  usually  seen  in  such  cases  the  ordinary 
sampler  make  a  cut  around  the  back  of  the  drift  from  A 
to  B,  say,  8  ft.,  whereas  the  vein  would  not  be  over  6  ft. 
wide,  and  so  reported.  To  accurately  sample  such  a  sec- 
tion is  well-nigh  impossible,  and  so  it  is  often  advisable 

— 104 — 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

to  shoot  off  the  wings  on  the  walls,  preparatory  to 
sampling. 

Taking  the  Sample. — A  moil  and  hammer  are  the  l)est 
all-round  tools  for  the  sampler,  though  a  heavy  pick  in 
medium  ground  will  usually  give  a  thoroughly  even  cut 
and  hence  a  correct  average.  A  box  to  catch  the  cuttings 
is  perhaps  as  good  and  convenient  as  anything ;  it  should 
not,  however,  be  much,  if  any,  wider  than  the  cut,  par- 
ticularly in  vuggy  or  porous  ground,  as  the  pounding  may 
shake  loose  (and  into  a  larger  box)  more  or  less  fine  and 
possibly  rich  material,  from  outside  the  sample  cut,  and  so 
salt  the  sample.  Where  this  sort  of  thing  is  suspected,  it 
is  best  to  use  one's  hand  to  remove  or  catch  the  chips  or 
rocks  as  they  are  loosened  by  the  moil. 

Where  two  sets  of  samplers  are  employed,  I  invariably 
work  them  together,  giving  one  the  odd  and  the  other  the 
even  numbers;  this  helps  to  eliminate  the  personal  ele- 
ment, and  tends  towards  uniformity  of  practice.  It  is 
also  safer. 

Checking  the  Sampling. — It  is  always  expedient  to  re- 
sample  a  certain  portion  of  the  cuts,  say,  from  5  to  10  per 
cent,  varying  on  the  importance  of  the  work.  Where  the 
examining  engineer  is  depending  on  assistants  to  do  the 
regular  sampling,  these  check  samples  should  be  his 
especial  care.  When  samples  cannot  be  assayed  at  the 
mine  or  in  the  immediate  vicinity,  the  engineer  is  at  a 
considerable  disadvantage.  In  such  cases  he  must,  as  a 
safeguard,  take  a  large  number  of  check  samples,  par- 
ticularly of  the  more  important  cuts,  say,  not  less  than  20 
per  cent ;  otherwise  he  may  have  to  pay  a  second  visit  to 
the  property  for  the  purpose  of  checking  up  some  of  the 
sampling,  which,  from  the  look  of  the  charts,  may  not 
conform  to  other  work  and  may,  or  may  not,  be  correct. 
In  most  important  mine  examinations  an  assayer  forms 
one  of  the  party,  and  the  samples  are  assayed  on  the 
ground.  In  such  cases  check  samples  should  be  taken  im- 
mediately the  work  is  started.  These  thoroughly  test  the 
whole  process  of  sampling  in  the  mine,  the  cutting  down 
of  the  samples,  and  also  the  assaying.    It  is  only  such  work 

— los— 


SAMPLING    AND    LSTI.MAIION    OF    ORE  JX    A    MINE 

as  this,  checking  the  original  assay,  that  gives  one  confi- 
dence in  the  methods  employed,  and  at  the  same  time 
serves  as  a  perfect  safeguard  against  salting. 

When  all  possible  precautions  are  taken,  two  samples 
from  the  same  cut  across  the  vein  will  seldom  check  with 
that  exactness  expected  from  original  and  duplicate 
samples  taken  by  sampling  machines  from  a  lot  of  crushed 
ore.  In  a  recent  examination  I  decided  to  re-sample  in  the 
original  cuts  two  strips  of  poor  ground  occurring  in  shoots 
of  pay  ore.    Here  are  the  results : 


Orig.  sample. 

Re-sample. 

Orig.  sample. 

Re-sample. 

$2.54 

$3.54 

$1.17 

$1-39 

2.30 

1.50 

1-57 

1.39 

2.71 

2.57 

1.24 

1.84 

2.18 

3.35 

1.68 

1.50 

1.98 

1.35 



1.76 

0.88 

Average, 

$1.41 

$1.53 

1.27 

1.25 

1.26 

1.87 

2.32 

2.60 

Average,   $2,035  $2.10 

It  will  be  noticed  that  while  some  of  the  assays  run 
higher  on  re-sampling,  others  are  lower,  yet  the  averages 
of  the  nine  origmals  and  duplicates  check  to  within  3  per 
cent.  Thus  it  can  be  seen  that  averages  cut  quite  a  figure 
in  mine  sampling,  hence  many  small  samples  are  prefer- 
able to  a  few  large  ones.  In  another  case,  where  the 
average  values  were  running  in  the  neighborhood  of  $240, 
one  sample  over  3  ft.  wide  assayed  $1,258;  a  re-sample 
gave  $1,208,  and  yet  another  $1,296,  the  average  being 
$1,254,  and  the  variations  about  4  per  cent  from  the 
original  sample.  I  have  another  recent  case  in  mind  which 
will  serve  to  illustrate  more  than  one  point:  A  line  of 
check  samples  along  a  practically  barren  portion  of  the 
vein  gave,  as  was  expected,  a  series  of  very  low  assays. 
One,  however,  ran  $25.41,  and  a  re-sample  was  called  for. 
The  same  man  took  both  samples,  but  with  the  re-sampie 
he  sent  this  note:  "Sample  probably  salted;  cut  a  small 
vug  and  fine  ore  ran  out  into  the  sample  box."  It  was 
salted.     The  assay  of  the  re-samiple  ran  $102.74.     I  then 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

sampled  the  cut  myself,  and,  noticing  a  small  seam  of 
quartz  containing  some  rich  silver  ore,  I  sampled  that  first, 
and  the  vein  matter  on  either  side  afterwards,  the  result 
being  as  follows :  8  ins.  of  $151.60  ore ;  i  ft.  3  ins.  of  $0.89 
ore ;  3  ft.  4  ins.  of  $4.47  ore ;  average,  5  ft.  3  ins.  of  $22.30 
ore,  practically  confirming  the  original  assay  and  showing 
that  in  certain  cases  the  box  as  a  means  of  catching  the 
samples  chipped  oflf  by  the  moil  is,  to  say  the  least, 
misleading. 

Sacking  the  Samples. — I  prefer  having  the  sample  sacks 
plainly  marked  on  the  outside  with  a  letter  to  designate 
the  series,  and  a  number,  but  not  necessarily  the  number 
of  the  sample.  One  can  then  see  at  a  glance  and  without 
opening  the  sacks  just  what  sample  it  is,  and  can  give 
them  to  the  assayer  in  any  order  one  may  choose,  prefer- 
ably, however,  in  serial  numbers.  The  check  samples, 
taken  as  the  work  proceeds,  also  receive  a  serial  number, 
having,  however,  no  relation  to  that  of  the  original  sample. 
The  small  samples  are  at  once  tied  up  in  their  sacks,  placed 
in  a  large  sack  and  taken  along  by  the  samplers  as  the 
work  proceeds. 

Cutting  Down  Samples. — Where  nothing  better  is  at 
hand,  a  board  platform,  covered  with  a  sheet  of  canvas, 
:is  described  by  Mr.  Rickard,  answers  admirably.  A  small 
rock-breaker  is,  however,  much  preferable,  and  with  it  at 
least  two  sets  of  riffles  and  pans  should  be  provided,  to 
the  end  that  the  samples  can  be  riffled  down  from  the  first 
in  a  rapid,  accurate  and  cleanly  manner,  and  all  coning, 
quartering  and  brushing  up  from  sheets  or  plates  abol- 
ished, and  with  them  many  sources  of  error.  For  this 
class  of  work  I  prefer  a  form  of  riffle  known  in  the  trade 
as  the  Jones  sampler,  the  safest,  neatest  and  most  expedi- 
tious apparatus  for  cutting  down  samples  that  I  know  of, 
and  one  that  I  have  used  for  several  years  with  perfect 
satisfaction.  The  sampler  consists  of  a  series  of  riffles, 
say,  y2  in.  wide,  but  instead  of  being  U-shaped,  as  the 
ordinary  riffle,  the  bottom  slopes  downwards  at  an  angle 
of  65°  (Fig.  3).  The  sampler  is  made  up  of  a  series  of  12 
or  more  riffles  of  uniform  width  (Fig.  4),  soldered  firmly 

— 107 — 


SAMPLING    AND    ESTIMATION    OF    ORE    !N    A    MINE 

together  so  that  the  spouts  are  alternately  right  and  left; 
thus  ore  that  enters  the  first  riffle  would  be  deflected,  say, 
to  the  right,  that  which  passes  the  second,  to  the  left,  and 
so  on.  It  is  supported  on  a  stand  (B)  and  the  metal  pans 
(C)  placed  under  the  discharge  from  the  riffle^  on  either 
side  (Fig.  3)  ;  a  small  shovel  with  turned-up  sides  (Fig. 
5)  is  provided  of  the  exact  width  of  the  riffles.  If  the 
sample  to  be  cut  down  is  crushed  in  a  laboratory  breaker. 


THE   JONES   SAMPLER. 


the  sampler  shovel  can  be  placed  to  receive  the  ore  as  it 
drc^s  out  of  the  jaws  of  the  machine ;  this  crushed  ore  is 
then  leveled  off  to  a  uniform  thickness  on  the  shovel  and 
gently  emptied  across  the  central  axis  of  the  riffles.  Should 
the  latter  consist  of  six  pairs,  then  one-half  the  sample 


SAMPLINCx    AND    ESTIMATION    OF    ORE    IN    A    .MINE 

will  fall  in  six  streams  into  the  right-hand  pan,  and  the 
remainder  in  six  streams  into  the  pan  at  the  left.  The 
sample  is  thus  divided  into  two  equal  parts,  the  result  of 
12  cuts  in  one  single  operation.  The  contents  of  one  pan 
is  taken  for  the  sample,  it  matters  not  which ;  the  other  is 
rejected.     The  sample  is  then  crushed  finer  and  the  op- 


/vc. -f 


ric.5 


F,G  S 


eration  repeated  after  the  sampler  is  brushed  out  and 
clean  pans  placed  in  position  below  the  riffle  spouts.  The 
operation  of  crushing  finer  and  passing  through  the  rif- 
fles is  repeated  until  the  sample  is  reduced  to  the  weight 
and  mesh  required.  It  will  be  noticed  that  no  sheet  or 
cloth  of  any  kind  is  required  nor  is  it  necessary  to  spread 


[09— 


SAMPLING   AND    ESTIMATION    OF   ORE   ^N    A    MINE 

the  ore  on  the  floor  or  on  plates,  that  the  ore  comes  in 
contact  with  metallic  surfaces  only,  to  which  precious 
metals  cannot  stick  or  become  entangled,  as  they  so  often 
do  in  canvas,  in  brushes  and  in  brooms ;  while  the  errors, 
the  labor,  and  the  superstition  of  "coning  and  quarter- 
ing" are  eliminated. 

All  samples  should  be  put  up  in  duplicate,  and  in 
such  manner  that  reliable  assayers  can  determine 
their  value  to  within  i  per  cent.  In  order  to  ac- 
complish this  desirable  end,  the  samples  should  be 
reduced  to  pass  at  least  a  loomesh  sieve,  and  this 
work,  be  it  remembered,  rightly  falls  to  the  sam- 
pler and  not  to  the  assayer.  The  latter  cannot  be  held 
responsible  unless  reliable  samples  are  furnished  him. 
Furthermore,  no  careful  engineer  could  afford  to  trust 
the  ordinary  assayer's  helper  to  cut  down  and  reduce  his 
samples.  It  is  only  when  the  samples  are  reduced  to  icx> 
mesh  and  put  up  in  duplicate  that  the  engineer  is  through 
with  his  part  of  the  business.  It  is  folly  to  divide  samples 
of  coarse  ore  and  send  them  to  rival  assayers  and  expect 
the  work  to  check ;  like  as  not,  both  will  be  wrong.  Where 
there  are  no  facilities  for  bucking  samples  at  the  mine,  and 
the  engineer  has  not  brought  the  necessary  apparatus  with 
him,  samples  can  be  reduced  to,  say,  20  or  30-mesh,  the 
finer  the  better;  they  should  be  put  up  in  stout  paper 
sacks  for  shipment  from  the  mine,  and  then  ground  and 
cut  down  in  the  office  of  the  assayer  in  the  presence  of 
the  engineer  or  his  representative.  Sampling  is  expen- 
sive. It  must  be  done  well  to  be  of  value,  and  in  no  stage 
of  the  proceedings  is  as  much  care  necessary  as  in  the 
final  one  of  cutting  down  the  samples.  This  work  is 
often  relegated  to  the  assayer's  assistant,  who,  while 
competent  for  ordinary  routine  work,  preparing  speci- 
men assays,  etc.,  yet  rarely  has  the  training  or  ability  to 
prepare  samples  for  assay  in  the  manner  that  smelters 
and  sampling  works  find  necessary  and  mining  engineers 
should  demand.  The  samples  taken  each  day  should, 
where  possible,  be  reduced  to  the  smallest  limit  before 
evening,  put  up  in  paper  sacks,  and  placed  under  lock. 


SAMPLINCi    AND    ESTIMATION    OF    ORE    IN    A    MINE 

Erratic  Samples. — The  proper  treatment  of  erratic 
samples  is  always  a  difficult  matter.  Mr.  Rickard  has 
given  both  sides  of  the  subject  very  careful  handling,  and 
little  more  can  be  said.  Where  re-sampling  confirms  the 
original  high  result,  the  inference  is  that  the  values  are 
there,  and  it  remains  to  make  the  proper  disposition  of 
them.  When  examining  for  an  intending  purchaser,  I 
usually  first  reject  the  erratic,  high  samples,  if  under  lo 
per  cent,  and  determine  the  average  value  of  the  remainder. 
Next,  I  insert  the  average  value  so  found  as  the  assumed 
value  of  the  higher  assays,  and  estimate  the  block  of  ground 
on  that  basis.  Where  erratic  samples  are  numerous,  their 
relative  importance  to  the  whole  number  of  the  samples 
must  be  taken  into  account,  as  well  as  the  amount  of 
ground  that  they  strictly  represent,  remembering  that  any 
given  block  of  ground  is  not  likely  to  maintain  as  high  an 
average  stoping  value  as  that  shown  by  sampling.  When 
erratic  samples  run  low  and  re-sampling  confirms  the  re- 
sult, they  must  stand.  The  whole  object  of  sampling  be- 
ing to  establish  the  average  value  of  the  ore,  it  is  well  to 
remember  that  it  is  not  an  exact  science ;  consequently,  in 
computing  results,  each  man  will,  to  some  extent,  be  gov- 
erned by  his  experience  in  determining  the  factors  to  be 
used,  so  that  his  final  estimates  will  conform  to  the  actual 
production  of  any  given  block  when  stoped  out.  This  is 
what  we  mostly  strive  for,  and  the  engineer  who  is  25 
per  cent  below  the  actual  value  in  his  estimates  of  "ore 
developed"  is  as  culpable  as  he  who  is  25  per  cent  above. 

In  a  former  issue  of  this  Journal,  I  attempted  to  de- 
scribe a  classification  for  ore  reserves  in  different  stages 
of  development,  and  also  for  ore  to  be  developed  in  the 
ordinary  course  of  mining.  These  definitions  have  been 
discussed  together  with  the  articles  on  mine  sampling, 
so  I  ask  the  same  liberty  of  reply.  "Ore  in  sight,"  I  am 
pleased  to  note,  has  no  defenders.  Air.  George  E.  Collins 
appears  to  take  exception  to  any  method  of  classifying  ore 
reserves,  but  gives  an  example  of  estimates  made  by  his 
brother  where  the  classification  I  suggested  is  very  nearly 
followed.     I  further  note  that  the  distinguished  engineer 

— Ill—- 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

for  whom  Mr.  Collins  made  this  classified  estimate,  bring- 
ing to  bear  a  wide  and  varied  experience  in  such  matters, 
unqualifiedly  indorses  my  classification,  using,  however, 
the  phrases  ''positive  ore,"  "probable  ore"  and  "possible 
ore"  for  those  suggested  by  me. 

Mr.  Collins  objects  to  my  definition,  "ore  developed" — 
exposed  on  four  sides — because,  forsooth,  some  such  blocks 
have  been  found,  in  certain  cases,  "to  enclose  a  large  bar- 
ren patch  in  the  center,"  though  why  it  should  be  in  the 
center  instead  of  any  other  part  of  the  block,  is  not  stated. 
Others  have  found  a  rich  patch  in  the  interior  of  a  block, 
and  can  with  equal  propriety  object  on  that  account.  These 
matters  have  nothing  whatever  to  do  with  the  proposed 
classification;  they  are  taken  care  of  in  the  factors  the 
engineer  employs  in  making  his  estimates  of  net  values. 
It  appears  to  me  that  Mr.  Collins  does  not  quite  under- 
stand the  significance  of  my  classification.  The  real  point 
is  not  seen  in  the  comparison  of  one  vein  with  another, 
but  in  the  various  stages  of  development  in  one  and  the 
same  vein.  Where  ore  exposed  on  four  sides  can  be  more 
safely  estimated  than  ore  hidden,  say,  on  two  or  more 
sides,  the  former,  being  a  safer  estimate,  is  properly  placed 
in  a  separate  class. 

The  geological  and  other  conditions  necessary  to  be 
taken  into  account  in  estimating  the  production  of  a  given 
block  of  ore,  and  applying  the  factors  of  correction,  are 
so  fully  treated  by  Mr.  Rickard  and  also  by  ]\Ir.  Puring- 
ton,  that  it  would  be  absurd  for  me  to  attempt  to  add 
another  word. 

Mr.  Collins  next  calls  attention  to  the  expense  of  open- 
ing out  a  mine  in  blocks  of  150  ft.  by  300  ft.  His  ideas 
are  apparently  connected  with  low-grade  veins,  and  not 
over  good  mining.  Taking  the  other  extreme  of  values, 
by  way  of  contrast,  I  have  seen  winzes  sunk  that  produced 
over  $1,000  worth  of  ore  per  foot  (in  one  case  $1,500), 
and  they  were  placed  at  less  than  50  ft.  apart.  Between 
these  extremes  the  blocks  that  I  suggested,  with  winzes  or 
raises  at  intervals  of  300  ft.  or  of  200  ft.,  will,  I  believe, 
fairly  conform  to  the  best  modern  practice.    The  ore-body 


SAMPLING    AND    ESTIMATION    OF    ORE   IN    A   MINE 

shown  in  Fig.  2  by  Mr.  Collins  is  not  typical  of  anything 
I  have  ever  seen,  and  such  is  the  opinion  of  others.  That 
an  ore-deposit  of  that  shape  may  occur  is  admitted,  but 
that  it  is  "typical  of  perhaps  the  largest  number  of  ore- 
bodies  in  fissure  veins"  is  denied. 

A  final  word  on  my  suggested  system  for  the  classifica- 
tion of  ore  reserves.  We  will  assume  the  various  blocks 
in  a  vein  have  been  measured,  sampled  and  carefully  ex- 
amined regarding  their  geological  environment,  and  that 
the  engineer  finally  decides  that  a  factor  of  10  per  cent 
should  be  applied  as  a  deduction  from  the  values  shown 
in  the  blocks  exposed  on  four  sides ;  then,  everything  being 
equal,  this  factor  of  safety  should  be  something  like  15 
per  cent  for  blocks  with  one  concealed  side,  20  per  cent 
where  two  sides  are  concealed,  and  something  like  33  per 
cent  where  only  one  face  of  the  ore  is  exposed.  While  on 
development  these  partially  exposed  ore  blocks  may  equal, 
or  even  exceed,  the  averages  found  in  the  fully  developed 
blocks,  still  the  chances  are  that  they  will  not,  and  a 
careful  engineer  will  limit  his  risks  in  a  manner  some- 
thing similar  to  the  above  factors,  if,  indeed,  he  will  be 
willing  to  concede  any  estimate  for  ore  exposed  only  on 
one  face. 

Philip  Argall. 
Denver,  Colo.,  May  28,  1903. 


The  Editor : 

Sir. — It  would  seem  from  Mr.  Philip  Argall's  refer- 
ences, in  your  issue  of  June  13,  to  my  criticisms  of  his 
former  article,  that  we  are  arguing  somewhat  at  cross- 
purposes.  I  certainly  did  not  object  to  the  classification 
of  ore  estimates.  What  I  do  deprecate  is  the  adoption 
of  cast-iron  definitions  for  such  classes,  which  will  not 
admit  of  variation  with  varying  conditions.  To  illustrate : 
I  object  to  using  the  same  expressions  to  denote  estimates 
of  ore  in  a  pockety  fissure-vein  and  a  comparatively  regu- 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

hr  impregnated  bed,  merely  because  both  are  exposed  on 
one,  two  or  more  sides,  as  the  case  may  be. 

I  did  not  suggest  that  m}-  brother  was  the  originator 
of  the  division  of  ore-reserves  into  the  three  classes  of 
''positive,"  ''probable"  and  ''possible"  ore.  I  believe  he 
adopted  the  idea  from  Mr.  Arthur  Winslow,  who  has 
made  use  of  three  similar  classes  in  his  Liberty  Bell  an- 
nual reports  for  years  past.  Mr.  Winslows  however, 
using  his  own  discretion  and  relying  on  his  personal  judg- 
ment as  to  the  mine  he  is  dealing  with,  is  far  less  exacting 
in  his  requirements  as  to  number  of  sides  exposed  than 
Mr.  Argall,  Mr.  Lawrence,  or  the  late  Mr.  Arthur  L.  Col- 
lins. He  carries  out,  in  fact,  the  principle  that  "the 
amount  of  evidence  to  be  required  when  making  estimates 
of  ore-tonnage,  and  the  number  and  nature  of  classes  into 
which  estimates  should  be  divided,  must  depend  on  our 
general  conclusions  as  to  the  nature  and  permanence  of 
the  ore-bodies  in  the  mines  under  consideration,  the  dis- 
tance between  the  workings,  etc." 

In  figuring  the  ore-body  No.  2  in  my  letter  I  did  not 
mean  to  imply  that  that  particular  shape  was  typical,  but 
that  similarly  irregular  ore-bodies  are  "typical  of  perhaps 
the  largest  number  of  ore-bodies  in  fissure-veins."  The 
regular  ore-shoots  of  the  text-books,  in  fact,  are  not  very 
common  in  the  field. 

I  think  Mr.  Argall  may  fairly  be  asked  to  explain  what 
he  means  w^hen  he  speaks  of  my  ideas  being  apparently 
connected  with  "not  over-good  mining."  It  will  be  re- 
membered that  I  was  discussing  the  mining  of  irregularly- 
occurring  ore-bodies  in  fissure-veins,  ars  to  which  I  will 
venture  to  state  two  propositions : 

1.  It  is  not,  necessarily  or  usually,  good  mining  to  open 
up  such  a  vein  in  blocks  of  150  ft.  by  300  ft.  Each  case 
must  be  decided  on  its  own  merits. 

2.  Any  expenditure  in  the  working  of  such  deposits 
beyond  the  minimum  necessary  to  discover  and  extract 
them  may  assist  good  experting.  It  is  not  part  of  good 
mining. 

I  did  not,  however,  object  to  the  expense  of  opening  up 
— T14— 


SAMPLING    AND    ESlIMATION    OF    ORE    IN    A    MINE 

a  mine  "in  blocks  of  150  ft.  by  300  ft.,"  but  to  expense  in 
raises,  etc.,  for  the  sole  purpose  of  blocking  out  irregular 
ore-bodies,  and  not  necessary  for  their  economical  ex- 
ploitation; working  mines,  in  fact,  for  the  best  showing 
rather  than  for  the  greatest  profit. 

Are  we  not  all  apt  to  forget,  at  times,  that  mining  engi- 
neers exist  only  for  the  sake  of  the  mines,  and  not  vice 
versa? 

George  E.  Collins. 

Denver,  Colo.,  June  22,  1903. 


The  Editor : 

Sir. — The  article  by  Mr.  Argall,  in  your  issue  of  Feb- 
ruary 14,  on  the  merits  or  demerits  of  the  old-fashioned 
expression  of  "ore  in  sight"  is  of  great  interest  to  the 
profession,  and  the  result  of  the  different  views  which  it 
will  bring  forward  should  induce  a  change  in  terms 
such  as,  it  appears,  is  very  much  needed. 

Taking  advantage  of  the  opportunity  of  discussing 
this  subject,  I  would  suggest  the  following  terms : 

Ore  Blocked, — Such  bodies  as  are  accessible  on  all 
sides,  that  can  be  accurately  measured  and  thoroughly 
sampled,  and  the  values  of  which  can  be  reliably  de- 
termined. 

Ore  Partly  Blocked, — Those  bodies  that  are  only  par- 
tially developed,  and  the  values  of  which  can  only  be 
approximately  determined. 

Ore  Faces. — Would  designate  those  bodies  that  are 
exposed  on  one  side  or  sh'ow  only  one  face,  and  whose 
values  can  only  be  determined  in  a  prospective  manner 
as  deduced  from  the  general  condition  of  the  mine  or 
prospect. 

G.  M.  GOUYARD. 

Denver,  Colo.,  Feb.  20,  1903. 


The  Editor: 

Sir. — Referring  to  the  discussion  regarding  "Ore  De- 
veloped," kindly  allow  space  for  this  bit  of  experience 
from  actual  practice. 


—115— 


SAMPLING   AND    ESTIMATION    OF    ORE    IN    A    MINE 

There  had  been  blocked  out  on  three  sides  a  piece  of 
ground  as  shown  in  sketch.  The  result  of  sampling 
showed  the  block  to  be  payable  ore,  though  with  but  a 
small  margin  of  profit  remaining  above  mining  and  mill- 
ing expenses.  Upon  stoping  the  ground,  there  was  en- 
countered a  pocket  of  ore  within  6  ft.  of  the  upraise  No. 
2^  which  carried  a  value  three  times  that  of  the  result  of 
sampling  the  block,  and  which  amounted  to  about  one- 
third  of  that  of  the  whole  piece  of  ground. 


.'^'-^^"Vitifk- 


n. 


y 


P.^o^-^"* 


A 


L 


This  is  a  reverse,  though  more  desirable  case  than 
the  ones  referred  to  in  Mr.  Collins'  article  of  February 
24,  in  which  the  ground  might  have  been  poorer  instead 
of  richer.  It,  however,  illustrates  the  same  principle  of 
uncertainty  with  which  we  have  to  deal. 

John  C.  Treadwell. 

Sahuayacan,  Mexico,  April  6,  1903. 


The  Editor: 

Sir. — I  was  very  much  interested  in  an  article  by  Mr. 
Philip  Argall,  in  your  issue  of  February  14,  in  regard 
to  the  use  of  the  term  "ore  in  sight.'*  What  Mr.  Argall 
has  to  say  is  very  much  to  the  point,  and  quite  agrees 
with  my  views  upon  this  subject. 

I  would  use  the  following  terms  to  qualify  and  more 
fully  explain  the  purely  general  expression  of  "ore  in 
sight" : 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

1.  Positive  Ore. — Ore  exposed  on  all  sides. 

2.  Probable  Ore. — Ore  exposed  on  two  or  three  sides. 

3.  Possible  Ore. — Ore  below  the  lowest  level,  or,  as 
Mr.  Argall  expresses  it,  "below  the  last  visible  ore." 

The  use  of  these  three  simple  words  is  recommended 

to  engineers. 

Benj.  B.  Lawrence. 

New  York,  Feb.  21,  1903. 


The  Editor : 

Sir. — Mr.  Collins  now  defines  his  position  as  objecting 
to  "the  same  expressions  to  denote  estimates  of  ore  in  a 
pockety  fissure-vein  and  a  comparatively  regular  impreg- 
nated bed,  merely  because  both  are  exposed  on  one,  two  or 
more  sides."  Here  we  differ  radically,  assuming,  for  th^ 
sake  of  argument,  such  an  unusual  phenomenon  as  a  "com- 
paratively regular  impregnated  bed" ;  the  engineer  would 
use  a  lower  factor  of  correction  in  dealing  with  the  latter 
than  in  the  case  of  a  pockety  fissure-vein,  to  the  end  that 
in  either  case  his  estimates  may  be  as  near  correct  as  he 
can  get  them.  So  long  as  mines  are  developed  by  levels 
and  winzes,  or  the  counterpart  of  the  latter — raises — the 
present  method  of  estimating  ore  reserves  in  blocks  will 
in  all  probability  prevail  and  a  block  with  all  sides  exposed 
will  also  receive  a  higher  estimate  than  those  with  one  or 
more  concealed  sides.  In  my  definition  "ore  developed," 
I  stated  that  as  a  general  rule  "the  smaller  the  blocks  of 
developed  ore  the  more  accurately  can  their  contents  be 
estimated" ;  suggesting  as  a  safe  rule  in  such  matters  that 
the  leng-th  of  a  block  should  not  exceed  twice  its  depth. 
Within  the  limits  I  gave,  there  is  nothing  very  rigid  about 
this.  Then  it  must  not  be  forgotten  that  other  classifica- 
tions are  provided  to  cover  cases  of  incomplete  develop- 
ment ;  consequently  I  cannot  see  where  the  so-called  "cast- 
iron"  definition  comes  in^  unless  it  is  the  requirement  that 
first-class  reserves  should  have  no  concealed  sides. 

It  is  good  business,  and  good  mining  also,  to  know, 
within  the  limits  of  commercial  accuracy,  what  materials 


-117— 


SAMPLING    AND    KSTIMATION    OF    ORE    IN    A    MINE 

or  ore  one  can  safely  depend  on ;  taking^  stock  does  not 
directly  increase  the  profits  in  any  business,  yet  it  is 
usually  considered  imperative  at  certain  intervals.  Turn- 
ing- now  to  the  hypothetical  ore-body  (Fig.  2  of  Mr.  Col- 
lins) to  which  I  am  again  referred,  I  desire  to  state  that 
there  is  no  evidence  in  the  sketch  to  justify  one  in  assum- 
ing that  the  outer  lines  drawn  by  Mr.  Collins  to  represent 
the  boundary  of  the  ore-body  have  any  foundation  in  fact. 
Considering  the  peculiarity  of  the  assumed  form  of  the 
ore-body  and  the  development  g^ven,  I  w^ould  construct  the 
outline  of  pay-ore  as  shown  by  the  hatching,  and,  while 
this  would  give  about  60  per  cent  less  than  Mr.  Collins 
finds,  I  believe  it  is  as  much  as  a  careful  engineer  dare 
allow  on  the  development  outlined. 

Philip  Argall. 
Denver,  Colo.,  July  11,  1903. 


UNSYSTEMATIC   SAMPLINa 

The  Editor: 

Sir. — I  have  been  much  interested  in  your  articles  on 
the  systematic  sampling  of  mines.  May  I  contribute 
a  few  remarks  upon  the  un-systematic  sampling  of  mines 
in  order  to  point  out  that,  like  all  other  good  methods, 
systematic  sampling  has  its  limitations  ? 

That  these  limitations  are  not  generally  recognized 
is  indicated  by  the  fact  that  certain  large  mine  invest- 
ment companies  are  reputed  to  go  so  far  as  to  formulate 
rules  by  which  their  scouting  engineers  are  bound. 
Two  such  rules  I  have  encountered  more  than  once 
and  they  are  substantially  as  follows : 

1.  No  ore  shall  be  considered  as  *'in  sight"  unless  it 
can  be  sampled  at  regular  intervals  on  at  least  three 
sides. 

2.  A  purchase  is  not  to  be  recommended  unless  there 
is  a  fixed  percentage  of  the  purchase  price  in  sight. 

Now,  I  think,  the  making  of  any  such  rules  as  these 
is  open  to  criticism  from  a  business  standpoint.  They 
are  not  conducive  to  the  acquiring  of  good  mining  in- 

—118— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

vestments.  I  can,  in  fact,  recall  an  instance  where  a 
good  opportunity  for  a  profitable  purchase  was  lost  in 
this  way,  and  I  have  no  doubt  many  of  my  readers  can 
recall  others. 

Thfe  scheme  of  systematic  sampling  is  based  on  the 
idea  that  mineral  deposits  occur  in  definite  and  measur- 
able shapes,  as,  for  instance,  a  fissure  vein  or  a  mineral- 
ized stratum,  but  we  know  from  experience  that  a  very 
large  percentage  of  all  mineral  deposits  are  most  irregu- 
lar in  shape  and  size,  and  in  such  deposits  it  seems  to  me 
that  systematic  sampling  is  apt  occasionally  to  be  actu- 
ally misleading.  By  systematic  sampling  I  mean  the 
taking  of  samples  across  the  lode  at  regular  intervals  on 
all  the  accessible  drifts  and  entries  with  a  view  to  figur- 
ing out  the  ore  in  sight  from  the  measurements  and  as- 
says thus  obtained. 

That  there  are  limitations  to  this  system  can  be  shown 
by  recalling  a  few  irregular  ore-bodies.  Take,  for  in- 
stance, the  Richmond  mine  at  Eureka,  Nevada;  the 
Copper  Queen  mine  at  Bisbee,  Arizona,  or  the  Cortez 
mine  at  Cortez,  Nevada,  all  of  them  mines  which  are 
identified  with  bodies  of  great  value  and  magnitude,  but 
of  such  erratic  form  that  I  cannot  see  how  the  ordinary 
methods  of  sampling  could  be  appHed  at  all ;  or  consider 
the  countless  silver-lead  deposits  wWch  occur  as  con- 
nected, or  partially  connected,  chambers  in  limestone. 
How,  may  I  ask,  is  one  to  figure  the  ore  in  sight,  accord- 
ing to  rule,  in  such  cases? 

Adopting  Mr.  Philip  Argall's  very  valuable  sugges- 
tions as  outlined  in  your  paper  of  February  14,  and  ap- 
plying them  literally,  one  could  figure  no  "ore  devel- 
oped" in  this  class  of  deposits.  Even  blocks  developed 
on  four  sides  are  like  as  not  to  be  solid  country  rock  in 
the  center,  and  yet,  in  mines  of  this  class  there  are  fre- 
quently hundreds  of  feet  of  development  with  ore  on 
from  one  to  four  sides  of  the  passages.  It  is  obviously 
most  unjust  to  say  tMs  is  all  ore  ''being  developed"  or 
ore  "expectant,"  and  it  should  not  be  forgotten  that 
these  pockety  and  irregular  deposits  have  contributed 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

very  largely  to  the  world's  supply  of  metals,  and  have 
produced  some  phenomenal  dividend  payers.  It  seems 
to  me  that,  in  cases  of  this  kind,  rules  must  be  in  a  large 
measure  disregarded,  and  the  value  of  the  engineer's 
report  will  depend  entirely  on  the  man  and  not  on  the 
system. 

Take  again  the  matter  of  outcrops;  gold  outcrops 
often  assay  more,  and  the  outcrops  of  silver,  lead  or  cop- 
per veins  often  assay  less  than  the  average  of  the  shoots 
beneath.  The  outcrops  of  the  famous  Nacozari  cop- 
per mine  was  a  large  mass  of  iron  gossan,  showing  only 
a  little  copper  carbonate  here  and  there.  The  big  cop- 
per mines  at  Cananea  presented  a  similar  appearance  on 
the  surface.  Obviously,  in  cases  of  this  kind  the  judg- 
ment must  be  guided  much  less  by  sampling  than  by 
mineralogical  and  geological  indications. 

What  alternative  is  there  to  take  the  place  of  syste- 
matic sampling?  The  question  is  hard  to  answer,  except 
in  a  general  way  that  the  method  of  sampling  must  be 
fitted  to  the  individual  case.  Mines  are  such  unsystematic 
things  at  best  that  no  two  will  stand  exactly  the 
same  treatment,  and  as  I  said  at  the  outset,  this  contri- 
bution is  intended  to  accentuate  the  limitations  of  sys- 
tematic sampling,  and  it  is  obviously  impossible  to  pre- 
scribe any  system  for  a  procedure  which  is  essentially 
un-systematic,  but  I  will  give  two  examples  which  I  hope 
will  illustrate  my  point,  and  I  can  show  that  even  in 
those  cases  where  systematic  sampling  fails  us  there  are 
still  avenues  of  procedure,  which,  if  they  fail  to  give  the 
concise  data  which  are  desirable,  do  at  any  rate  provide 
a  way  by  which  a  fair  estimate  can  be  made  of  a  mine. 
In  both  cases  the  examinations  were  made  by  mining 
engineers  who  take  the  highest  rank  in  the  profession. 

The  first  case  was  that  of  sampling  a  large  gold  mine 
in  Mexico.  Th'e  engineer  spent  two  days  poking  around 
the  mine  with  a  sampling  pick,  studying  the  vein  and 
the  ore-occurrence.  He  put  in  the  evenings  studying 
the  shipments  and  acquiring  general  information  con- 
cerning the  conditions  of  the  locality.  Now,  in  studying 


SAMPLING   AND    ESTIMATION    OF    ORE    IN    A    MINE 

the  vein  he  found  that  there  were  three  or  four  distinct 
kinds  of  ore  exposed  in  th'e  workings  and  his  wide  ex- 
perience in  such  matters  enabled  him  to  make  a  close 
mental  estimate  of  the  respective  amounts  of  such  ore 
in  sight,  so  it  only  required  a  few  samples  of  each  kind 
of  quartz  to  give  him,  at  one  stroke,  data  from  which  he 
could  form  a  very  fair  estimate  of  the  mine.  These  sam- 
ples were  taken  and  assayed  under  his  directions,  and 
I  know  that  the  returns  from  them  led  him  to  a  correct 
opinion  of  the  mine ;  for,  although  he  at  once  proceeded 
to  sample  the  mine  in  a  systematic  manner,  the  vein 
being  one  which  yielded  itself  readily  to  such  an  opera- 
tion, and  although  he  and  I  kept  in  touch  with  the  mine 
until  its  decline,  he  never  had  occasion  to  change  his 
opinion. 

The  above  example  shows  how  a  man  gifted  with 
good  judgment  can  get  at  the  facts  without  systematic 
sampling,  but  it  is  also  an  example  of  the  fact  that  no 
thorough  engineer  will  ever  neglect  so  valuable  an  aid 
as  systematic  sampling  wherever  such  a  method  is  at  all 
applicable.  By  treating  the  various  mineral  constituents 
of  a  lode  we  find  a  way,  hardly  a  system,  by  which  much 
valuable  information  may  be  gained.  Every  mine  shows 
different  kinds  of  ore,  and  very  often  they  are  of  such  a 
nature  that  having  once  educated  the  eye,  by  assaying 
the  various  kinds,  it  is  entirely  possible  to  distinguish 
the  different  grades  on  sight.  In  deposits  of  very  erratic 
shape  the  information  to  be  gained  in  this  way  may  be 
very  considerable,  and  may  lead  to  a  comprehension  of 
the  deposit  which  will  facilitate  systematic  sampling 
afterwards. 

The  second  case  was  that  of  a  large  copper-silver  de- 
posit which  I  sampled  with  the  other  of  the  two  engi- 
neers previously  alluded  to.  Here  the  ore-bodies  left 
standing  were  so  shapeless  that  the  ordinary  methods 
of  sampling  and  measurement  were  utterly  useless.  He 
went  at  once  to  the  pith  of  the  matter  by  taking  a  few 
general  samples,  examining  them  carefully  and  separat- 
ing them  by  concentration  into  their  constituent  parts. 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

namely,  clean  concentrates,  middlings,  slimes  and  tail- 
ings. These  various  parts  were  each  carefully  inspected, 
accurately  weighed  and  assayed,  and  with  the  knowledge 
gained  thereby  he  was  able  to  go  through  the  mine  and 
decide  correctly,  not  only  as  to  what  was  ore  and  what 
was  waste,  but  he  could  pass  upon  the  concentrating 
qualities  of  the  material.  This  method  of  sampling 
showed  the  instincts  of  the  metallurgist.  The  location 
of  the  mine  was  such  that  concentration  was  the  only 
practicable  treatment.  Like  the  last  example,  this  was 
a  case  of  educating  the  eye,  but  it  differed  in  that  the 
eye  was  educated  to  read  in  values  and  percentages  of 
concentrates  as  well  as  in  values  of  crude  ore.  To  this 
experienced  metallurgist  it  was  a  useful  "short  cut"  to 
results,  but,  like  all  short  cuts,  I  imagine  it  might  be 
dangerous  to  a  beginner.  The  question  of  structural 
geology  was  also  interesting  in  this  case.  The  owners 
considered  the  deposit  a  large  chimney,  and  worked  it 
with  the  idea  that  the  ore  went  down,  found  it  did  not 
go  below  a  certain  level,  let  the  mine  partly  fill  with 
water  and  put  it  on  the  market  with  the  usual  tale  of 
inadequate  pumping  machinery.  The  deposit  proved 
to  be  a  large  anticlinal  fold  with  the  axis  dipping  slightly 
to  the  east.  There  was  more  ore  and  more  water  in  the 
eastern  part  of  the  mine  than  anywhere  else,  and  the 
chance  that  the  ore  might  continue  along  the  axis  seemed 
tempting  enough  to  justify  a  little  work.  This  interest- 
ing question,  however,  has  not  been  completely  solved 
as  yet. 

Another  case  will  serve  to  illustrate  my  theme.  I 
lately  investigated  a  large  low-grade  deposit  in  Colorado. 
The  lode  is  a  brecciated  band,  in  a  schist  country,  about 
200  ft.  wide  and  two  or  three  times  as  long.  In  this  case 
also  the  structural  geology  is  far  more  interesting  and 
vital  a  question  than  the  sampling,  but  as  this  is  an  arti- 
cle on  unsystematic  sampling,  I  will  speak  only  of  the 
conditions  which  affected  that  part  of  the  work.  The 
brecciation  was  complete  and  coarse;  by  this  I  mean 
that  large  open  spaces  had  been  formed  as  would  occur 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

in  a  pile  of  rocks.  Some  of  these  spaces  were  filled 
nearly  solid  with  chalcopyrite  which  carried  $35  in  gold 
per  ton  and  many  others  had  only  a  little  chalcopyrite, 
the  remainder  of  the  filling  being  a  loose  sinter,  and 
both  it  and  the  schist  contained  very  little  of  value.  In 
fact,  50  per  cent,  of  the  values  lay  in  these  nodules  of 
copper  pyrite.  As  the  mine  had  been  worked  by  lessees 
it  was  only  natural  that  the  surfaces  exposed  under- 
ground were  destitute  of  copper  nodules.  Here  was  a 
case  where  systematic  sampling  with  a  moil  and  ham- 
mer would  result  in  error,  but  where  the  method  of  talc- 
ing samples  by  car-load  lot  and  shipping  the  same  to 
the  smelter  would  give  useful  information. 

I  cannot  resist  the  temptation  to  say  a  few  words  in 
regard  to  the  method  of  sampling  "by  car-load  lot." 
One  cannot  but  feel  a  little  cheap  and  inefficient  with  a 
sackful  of  carefully  reduced  little  samples  all  carefully 
tied  up  and  sealed,  when  confronted  by  the  man  who 
believes  in  "sending  a  car-load  to  the  smelter  and  get- 
ting your  money  for  it."  Somehow  this  "sending  a  car- 
load to  the  smelter  and  getting  your  money  for  it"  has 
such  a  grand  and  practical  sound  to  it,  that  it  impresses 
people.  But  let  us  follow  the  car-load  to  the  smelter 
and  see  what  is  done  with  it.  Do  they  take  a  nice,  clean 
furnace  and  smelt  that  car-load  separately  and  weigh 
up  the  values  from  the  results?  Oh,  no;  they  just  put 
it  through  a  sampler,  which  often  is  none  too  clean  or 
too  carefully  handled,  and  reduce  the  car-load  to  one 
little  assay  sample.  It  is  a  noticeable  thing  also  that 
the  man  who  beheves  in  taking  his  samples  by  the  car- 
load is  generally  the  same  man  that  believes  in  hand- 
sampHng  at  the  smelter  in  preference  to  machine  sam- 
pling. So  that  the  man  who  does  his  sampling  by  car- 
load lots  does  his  buying  on  one  assay  sample,  while 
the  other  man  does  his  buying  on  th'e  results  of  many 
dozen  assay  samples,  and,  I  believe,  that  each  one  of 
the  many  may  be  made  as  truly  representative  as  the 
machine-sampled  car-load. 

I  have  just  shown  an  instance  where  car-load  sam- 

—123— 


SAMPLING   AND    ESTIMATION    OF   ORE, IN    A    MINE 

pling  was  needed,  but  in  the  majority  of  cases,  I  think, 
the  sampling  by  hammer  and  moil  is  preferable,  because 
it  can  be  made  to  yield,  not  only  the  average  value  of 
the  ore  in  a  given  block  of  ground,  but  the  quantities  and 
qualities  of  the  various  grades  as  well. 

I  hope  that  I  have  succeeded  in  calling  attention  to 
the  fact  that  systematic  sampling  is  only  one  of  many 
tools  at  the  engineer's  command.  Just  as  the  skilled 
mechanic  uses  now  one  tool  and  now  another,  when 
the  work  before  him  changes,  so  the  engineer  may  find 
it  advantageous  to  vary  his  methods. 

George  J.  Bancroft. 
Denver,  Feb.  17,  1903. 


ON  SAMPLING  THE  FLOOR  OF  A  WET  LEVEL. 

The  Editor: 

Sir. — I  have  read  the  articles  by  Mr.  T.  A.  Rickard  on 
sampling  of  ore  with  great  interest,  and  it  gives  me 
pleasure  to  contribute  some  data  upon  one  branch  of 
the  subject  which  has  lately  come  within  my  experience. 
I  refer  to  the  sampling  of  the  bottom  of  a  wet  level. 

This  is  by  no  means  a  pleasant  or  easy  branch  of  sam- 
pling; neither  can  the  sample  procured  be  expected  to 
be  as  accurate  as  one  taken  in  a  back  or  wall,  since  the 
ore  broken  along  the  groove  made  by  the  moil  remains 
more  or  less  in  place  until  collected  by  hand  and,  in 
doing  this  one  allows  many  of  the  smaller  pieces  to 
escape.  The  tendency,  therefore,  is  rather  to  obtain 
under-average  values,  because  the  richest  ore  is  usually 
the  most  brittle.  On  the  other  hand,  however,  the 
cracks  in  the  floor  of  a  level  form  a  good  receptacle  for, 
and  in  general,  become  filled  with,  the  rich  fines,  and 
this  would  ordinarily  tend  to  give  over-average  values; 
but,  in  the  case  to  be  described,  water  was  always  at 
hand,  and  by  washing  each  sample  carefully  this  source 
of  error  was  eliminated. 

All  samples  were  taken  in  the  usual  way,  by  chipping 
a  straight  groove  across  the  vein  with  the  aid  of  a  ham- 


SAMPLING   AND    ESTIMATION    OF    ORE    IN    A    MINE 

mer  and  moil.  In  places  we  tried  shooting,  but  found 
it  unsatisfactory.  The  flow  of  water  was  about  300  gals. 
per  minute,  nearly  all  of  which,  in  most  cases,  we  suc- 
cessfully dammed  back  and  carried  in  a  flume  over  that 
stretch  of  level  being  sampled.  The  grade  of  the  level 
was  usually  enough  to  keep  the  head  of  the  flume  higher 
than  the  discharge,  but  in  places  where  the  floor  was 
flat  or  where  it  sagged,  the  difflculty  was  overcome  by 
making  the  dam  one  plank  or  more  higher;  in  all  such 
cases  the  dam  should  be  kept  as  low  as  possible,  other- 
wise the  unnecessary  pressure  behind  it  will  accelerate 
leakage.  The  dam  consisted  of  a  2-in.  plank,  12  ins. 
wide  and  4  ft.  6  ins.  long,  the  level  being  timbered  with 
sets,  4  ft.  6  ins.  between  the  timbers,  at  5  ft.  intervals. 
The  mud  and  loose  rock  were  removed  from  the  sill,  and 
the  dam  spiked  on  it,  all  passages  for  water  being 
stopped  up  with  clay  procured  in  neight)oring  workings 
of  the  mine,  or  with  thick  mud  when  the  supply  of  clay 
gave  out.  Nailing  the  dam  to  the  side  of  the  sill  was 
tried,  but  this  failed  to  keep  out  the  water  as  effectually. 
The  flume  was  12  by  10  ins.  inside  cross-section,  and 
was  made  by  nailing  together  three  12  by  2-in.  planks  in 
sections  of  16  and  9  ft.,  the  longer  ones  being  used  for 
the  straight  parts  of  the  level  and  the  shorter  ones  for 
the  curves.  The  inlet  section  was  made  wedge-shaped, 
being  i  ft.  wide  at  its  lower  end  and  2  ft.  3  ins.  at  the 
dam  end;  the  latter  end  fitted  into  a  recess  sawn  in  the 
dam.  On  the  lower  end  of  each  section,  overlapping  it 
by  4  ins.,  was  nailed  a  collar,  made  of  two  14  by  8  by  2-in. 
planks  at  the  side  and  one  12  by  8  by  2  ins.  at  the  bot- 
tom, and  in  putting  up  the  flume  the  head  end  of  each 
section  was  laid  in  the  protruding  half  of  the  collar  on 
the  section  above  and  made  water-tight  with  clay.  At  first 
we  had  the  collars  made  of  i-in.  plank,  but  these  proved 
unserviceable,  because  they  were  apt  to  be  broken  by 
the  rough  handling,  accompanying  the  shifting  of  the 
flume.  To  give  the  flume  a  fairly  even  grade,  it  was 
blocked  up  underneath  with  timbers.  With  three  sam- 
plers, taking  samples  at  lo-ft.  intervals,  it  was  found  that 

—125— 


SAMPLINC;    AND    ESTIMATION    OF    ORE    IN    A    MINE 


— 126 — 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

a  Stretch  of  200  ft.  usually  required  a  full  day's  work; 
the  time,  of  course,  varied  with  the  local  conditions ;  in 
this  particular  case  the  veins  did  not  exceed  an  average 
width  of  2  ft. 

The  way  the  work  was  conducted  was  as  follows :  On 
the  previous  shift  a  gang  of  men  took  up  the  rails,  re- 
moved the  loose  dirt,  which  was  one  to  two  feet  deep, 
and  relaid  the  rails  so  that  the  flume  could  be  brought  in 
upon  timber  trucks.  The  rails  were  originally  spiked  to 
sleepers  above  the  sills^  but  were  relaid  on  the  sills  so 
that  the  track  was  lowered  6  to  12  ins.  The  flume  was 
then  laid  on  that  side  of  the  level  opposite  to  that  occu- 
pied by  the  vein.  The  places  to  be  sampled  were  then 
marked  off,  and  men  were  set  to  work  to  shovel  the  re- 
maining dirt  to  one  side  and  pick  down  trenches  as  far 
as  the  ore  in  place  and  bail  out  the  water  which  oozed 
through.  For  baiHng  they  used  tomato  cans.  This 
was  slow  and  tedious  work;  often  it  became  necessary 
to  build  a  small  clay  dam  on  that  side  of  the  trench  from 
which  the  water  came,  or  even  all  round  it,  or  else  to  dig 
another  trench  just  above  the  place  where  the  sample  was 
to  be  taken  and  bail  from  it. 

If  the  vein  be  of  such  a  nature  that  it  breaks  in  slabs 
parallel  to  its  strike,  an  accurate  sample  can  be  taken 
from  under  water;  if  not,  it  becomes  necessary  to  make 
the  trench  dry  because  the  muddy  water  prevents  one 
from  seeing  what  is  being  done. 

Should  the  vein  jump  from  one  side  to  the  other,  it  is 
advisable  to  take  all  samples  on  the  one  side  first,  then 
move  the  flume  and  take  those  on  the  other  side.  In  re- 
gard to  starting  the  samples  at  the  lower  end  of  the 
stretch'  first  and  working  upwards,  or  vice  versa,  the 
local  conditions  of  each  stretch  have  to  be  taken  into^ 
consideration.  As  regards  working  up  or  down  the 
level,  owing  to  the  removal  of  dirt  and  lowering  of  the 
track,  work  must  of  necessity  commence  at  the  lower 
end. 

The  first  day  we  used  a  15-in.  galvanized  iron  pipe  in 
place  of  the  flume,  but  found  it  unsatisfactory  for  many 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

reasons.  It  takes  too  much  time  to  put  the  sections  to- 
gether, also  the  pipe  cannot  be  made  to  follow  the  bends 
of  the  level  as  easily  as  a  flume  can.  It  cannot  be  sawn 
or  cut  to  any  desired  length  as  easily.  By  being  open 
along  its  whole  length  a  flume  possesses  another  great 
advantage  over  a  pipe.  In  the  case  of  a  cross-cut  con- 
ducting extra  water  into  that  stretch  of  level  being  sam- 
pled, a  clay  dam  with  a  4-in.  pipe  leading  into  the  flume 
was  found  sufficient  to  rid  that  part  of  the  level  of  the 
extra  inflow.  The  water  bailed  from  the  trenches  could 
be  poured  straight  into  the  flume,  whereas  with  the  use 
of  a  pipe  much  inconvenience  and  loss  of  time  was  occa- 
sioned by  the  water  bailed  from  the  trench  above  running 
down  and  filling  the  lower  ones.  The  cost  of  the  pipe 
was  95c.  per  foot,  and  that  of  the  flume  17c.  (timber  at 
the  mine  in  question  costing  $20  per  1,000  ft.).  Except 
in  the  matter  of  weight,  which,  on  account  of  the 
small  amount  of  moving  necessary,  is  of  no  moment, 
ii  is  obvious  that  the  flume  possesses  all  the  advantages. 

The  accompanying  sketch  shows  the  plan,  side  eleva- 
tion and  end  elevation  of  the  flume. 

Ernest  Lew. 
Denver,  Colo.,  Feb.  25,  1903. 


The  Editor : 

Sir. — Referring  to  Mr.  T.  A.  Rickard's  excellent  article 
in  the  Journal  of  February  14  on  mine  sampling,  permit 
me  to  oflfer  suggestions  on  a  few  points.  A  gad  made 
with  an  eye  and  fitted  with  a  handle  will  be  found  con- 
venient for  taking  samples  in  place  of  the  moil  Mr. 
Rickard  uses.  It  is  easier  held  in  a  variety  of  positions 
than  the  moil  and  more  rapid  work  can  be  done  with  it. 
I'OT  the  hardest  rock  the  moil  is  better.  For  spreading 
the  broken  up  sample,  when  coned  on  the  canvas,  a 
piece  of  thin  sheet  iron  or  tin,  about  4  by  8  ins.,  will  be 
found  convenient ;  with  it  the  same  sort  of  even  distribu- 
tion can  be  made,  by  going  round  and  round,  that  is  ob- 
tained  with  the  square-pointed   shovel  on  the  sampling 

—128— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

works  floor  with  larger  samples.  As  a  tag  for  the 
sacked  samples,  half  of  an  ordinary  manila  express  tag 
is  excellent.  The  number  is  put  on  in  duplicate  with 
pencil,  and  the  tag  folded  twice  and  put  into  the  sack. 
This  will  resist  any  amount  of  hard  usage  that  the  sacks 
will  stand.  Wooden  tags  sometimes  become  illegible 
when  samples  are  sent  a  long  distance  by  express.  In 
rich  or  otherwise  important  bodies  of  ore  a  good  safety 
device  is  to  divide  the  sample  at  the  mine  in  triplicate, 
give  each  portion  a  separate  number,  send  two  sets  to  the 
assayer  and  reserve  the  third  for  control.  This  makes  a 
check  both  on  the  assaying  and  the  sampling.  Only 
those  control  or  triplicate  samples  are  assayed  in  which 
discrepancies  between  the  original  and  duplicate  appear. 
This  device  is  specially  applicable  in  cases  where  the 
mine  is  remote  and  difficult  of  access  and  the  sacked  ore 
is  taken  away  by  the  sampler.  Th^e  use  of  it  has  saved 
the  writer  the  necessity  of  a  long  and  costly  return  trip 
and  a  re-sample.  A  discussion  from  Mr.  Rickard  of  the 
advisability  of  combining  several  cuts  across  the  vein  at 
short  intervals  into  one  sample,  or  of  taking  separate 
samples  at  greater  intervals,  and  in  what  cases  each  of 
these  methods  is  preferable,  would  be  interesting.  This 
is  a  matter  that  requires  judgment,  and  a  decision  on  it 
lias  much  to  do  with'  the  time  and  expense  required  to 
make  a  given  examination,  as  well  as  often  determining 
the  thoroughness  of  the  work. 

Chester  F.  Lee. 
Seattle,  Wash.,  Feb.  25,  1903. 


The  Editor: 

Sir. — I  have  read  your  article  on  sampling  with  much 
interest.  Personally,  however,  I  believe  in  what  you  call 
"grab  samples."  Experience  in  buying  ore  h!as  given  me 
a  fair  judgment  as  to  the  average  appearance  of  any 
special  lot.  In  veins  I  like  to  know  what  each  band  of 
particular  appearance  runs,  and  especially  I  want  the 
owner  of  the  mine  to  show  me  the  richest  part  of  the 


SAMPLING    AND    ESTIMATION    OF    ORE    IN,  A    MINE 

lode,  SO  that  I  may  know  its  proportionate  value  with 
the  rest  of  the  vein  matter,  from  which  I  take  grab  sam- 
ples. These  samples  act  as  checks  on  the  larger  average 
samples  taken.  Now  these,  when  put  away  in  my 
pocket,  have  twice   saved   me   from   being  ''salted." 

I  think  a  man  with  experience  can  tell  pretty  well  in 
most  cases  what  is  a  representative  piece  of  ore,  and  use 
it  as  a  check  on  the  average  samples  wMch  are  so  easily 
tampered  with. 

For  taking  samples,  I  believe  in  a  locked  mail  bag  of 
leather,  which  will  easily  show  punctures,  and  also  blank 
samples  already  prepared  to  put  in  with  the  samples 
taken  from  the  mine.  This  bag  when  taken  under- 
ground is  as  safe  a  guard  as  anything  can  be  against  the 
samples  being  tampered  withL 

Large  samples  weighing  a  ton  or  more  are,  of  course, 
best,  but  at  mines  unprovided  with  crushers,  help  is 
needed  to  crush  them  down,  which  increases  the  risk. 

One  man,  whom  I  know,  prepared  little  balls  of  clay 
containing  gold  washed  out  of  the  ore,  which  could  be 
thrown  from  quite  a  distance  into  the  pile  without  it 
being  likely  that  they  would  be  observed.  A  grab  sam- 
ple in  this  case  would  be  all  that  would  save  one  from  a 
false  estimate. 

For  young  men  your  advice  could  not  be  improved 
upon,  but  each  man  as  he  gains  experience  has  to  be  a 

law  unto  himself. 

W.  M.  Courtis. 
New  York,  March  7,  1903. 


The  Editor: 

Sir. — I  have  read  with  much  interest  the  articles  and 
discussions  on  mine  sampling  appearing  in  your  Journal. 
I  agree  with  Mr.  Courtis  that  specimen  or  grab  samples, 
representing  the  different  classes  of  ore  in  a  mine,  are  use- 
ful in  sizing  up  the  situation  underground,  and  some> 
times  serve  as  a  check  on  the  other  samples. 

In  sampling  a  vein  where  most  of  the  values  are  in  on§ 
—130— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

or  two  Streaks  of  friable  ore  and  the  remainder  hard 
quartz,  I  have  obtained  good  results  by  measuring  and 
sampling  the  rich  streak  separately  and  figuring  the 
average  grade  from  the  assays.  This  usually  saves  time 
in  sampling,  but  requires  another  assay. 

The  information  obtained  is  worth  the  cost  of  the  extra 
assay,  and  to  the  mine  operator  it  may  save  the  cost  of 
breaking  down  quartz  too  poor  to  pay,  although  the  aver- 
age across  the  vein,  thanks  to  the  pay  streak,  may  indicate 
that  all  should  be  mined. 

In  a  large  vein  there  may  be  a  streak  next  to  a  wall 
that  will  pay  to  mine,  while  the  rest  of  the  vein  may  be 
practically  barren.  A  sample  across  such  a  vein  would  be 
wrong,  and  might  indicate  that  the  vein  was  too  low 
grade  to  work. 

In  figuring  averages,  values,  etc.,  the  engineer's  slide 
rule  is  very  useful. 

Albion  S.  Howe. 

San  Francisco,  April  3,  1903. 


MINE  VALUATION. 

The  Editor : 

Sir. — In  reading  the  papers  on  ''Sampling  and  Estima- 
tion of  Ore  in  a  Mine,"  it  has  occurred  to  me  that  a  few 
remarks  on  South  African  practice  may  prove  of  interest 
to  your  readers,  and  in  submitting  them  I  am  encouraged 
by  Mr.  Rickard's  reference  to  the  influence  of  the  Rand 
on  gold  mining  practice  to  believe  that  American  engi- 
neers are  keenly  appreciative  of  methods  used  in  the 
Transvaal. 

No  two  gold-fields  are  exactly  the  same.  To  a  certain 
extent  each  is  a  law  unto  itself.  General  rules  must  be 
supplemented  by  a  thorough  knowledge  of  local  condi- 
tions. The  importance  of  sampling  varies  considerably  in 
different  localities.  For  instance,  suppose  there  is  a  huge 
deposit  of  low-grade  material  which  must  be  quarried 
from  a  mountain.  In  such  a  case  there  may  be  little  op- 
portunity for  selection.     On  the  other  hand,  if  you  have 

—131— 


SAMPLING    AND    ESTIMATION    OF    ORE    I^^    A    MINE 

to  deal  with  gold-bearing  deposits,  such  as  those  of  the 
Witwatersrand  beds,  then  sampling  becomes  of  vital  im- 
portance. In  some  districts  sampling  may  be  considered  a 
desirable  luxury ;  here  it  is  an  absolute  necessity. 

It  has  always  been  recognized  on  the  Rand  that  sam- 
pling was  of  the  utmost  importance,  but  the  mistake  was 
made  at  first  of  imagining  that  anyone  could  sample 
efficiently.  An  applicant  who  professed  ignorance  of  all 
mining  matters  was  looked  upon  as  a  proper  man  for  a 
sampler.  All  this  is  changed  now.  Whereas,  one  ineffi- 
cient man  was  once  considered  sufficient  for  sampling, 
many  of  the  big  properties  now  employ  at  least  two  good 
men,  who  give  their  entire  time  to  the  sampling  of  the 
underground  workings  and  the  keeping  of  the  records. 
Experience  has  shown  that  this  pays  well. 

As  is  well  known,  before  milling  commences  on  a  Trans- 
vaal gold  mine,  big  ore  reserves  are  blocked  out,  as  much 
as  350,000  tons  being  usually  developed  by  the  time  the 
stamps  commence  to  drop.  For  a  short  period  after  the 
shafts  first  cut  the  reefs,  the  samples  are  generally  as- 
sayed on  an  adjoining  mine,  or  in  the  town.  When  the 
samples  become  numerous,  the  mine  opens  its  own  assay 
office.  Sometimes,  in  the  development  stage  of  a  mine, 
one  man  is  employed  to  do  the  assaying  and  sampling, 
but  it  is  considered  better  to  keep  the  assay  and  the  samp- 
ling departments  separate.  More  often  a  man  is  engaged 
to  do  the  sampling  and  surveying.  As  the  mine  opens  out, 
more  men  are  required,  and  when  in  full  swing  a  prop- 
erty of  200  stamps  employs  a  surveyor  and  assistant, 
an  assayer  and  assistant,  and  a  sampler  and  assistant. 
These  six  men  are  responsible  for  the  mine  valuation. 

Before  the  war  a  fashionable  way  of  developing  a  mine 
was  to  run  what  are  known  as  intermediate  drifts  parallel 
with  the  main  levels.  In  Fig.  i  a  sketch  of  an  intermediate 
drift  is  given.  These  drifts  are  started  off  east  and  west 
from  the  raise ;  and  at  interv^als  of  35  ft.  or  so,  as  at  A  A, 
box  holes  are  blown  through  to  the  main  levels.  Not  only 
was  the  main  level  carefully  sampled,  but  the  intermedi- 
ate drift  as  well.    The  height  of  the  intermediate  drift  was 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 


made  one-half  that  of  the  level.  Individual  samples  vary 
considerably,  but  over  a  long  distance,  as  from  i  to  2, 
the  average  value  comes  out  about  the  same.  I  am  glad  to 
say  that  the  practice  of  running  these  intermediate  drifts 
is  dying  out,  for  they  are  needlessly  expensive. 

During  the  development  stage  of  a  mine  a  frequent 
method  of  procedure  is  to  sample  the  faces  of  the  drifts 
after  each  blasting  operation.  This  means  an  interval  of 
about  5  ft.,  which  is  considered  a  fair  distance  between 
samples.     The  advantage  of  sampling  the  mine  in  this 


Pkl-I 


manner  is  that  the  assay  will  show  whether  the  miner  is 
keeping  on  the  rich  seam,  or  "leader."  The  disadvantages 
are  that,  as  a  rule,  the  drift  at  the  face  is  nearly  choked  up 
with  broken  ore,  so  that  only  one  section  can  be  obtained, 
and  that  the  sampler  is  necessarily  hurried  by  the  miner, 
who  objects  to  the  delay  caused  by  him.  The  same 
method  is  pursued  in  winzes  and  raises,  samples  being 
taken  after  every  "round." 

Mr.  Rickard  g^ves  an  instance  of  three  samples  being 
taken  across  a  vein  12  ft.  wide.  On  the  Rand  a  sample 
over  such  a  big  width  would  not  be  taken,  but  the  12  ft. 
would  be  divided  oflf  into  sections  of  2  ft.,  and  each  sec- 
tion sampled  separately.  Look  at  Fig.  2,  and  you  will 
understand  why  samples  of  sections  are  taken.  In  this 
example  four  distinct  samples  would  be  taken,  one  from 
each  of  the  stringers,  as  it  is  of  the  utmost  importance  to 
find  where  the  gold  lies.  Suppose  stringer  No.  i  persis- 
tently assayed  2  to  3  dwts.,  or  much  below  the  limit  of 


—133- 


SAMPLING    AND    ESTIMATION    OF    ORE    15^    A    MINE 

pay-ore;  then  it  would  probably  be  decided  to  leave  this 
stringer  in  the  hanging  wall  and  narrow  up  the  stope. 
No  matter  if  the  stringers  are  thick  or  very  thin,  they 
are  invariably  sampled  separately.  On  some  mines,  the 
waste  rock  found  between  the  stringers  is  also  sampled 
and  assayed,  but  it  is  really  only  necessary  to  take  occa- 
sional samples  of  this  waste,  as  it  seldom  contains  gold  in 
payable  quantity. 

Let  us  suppose  that  there  is  an  unbroken  mass  of  aurif- 


FiG.  2 


erous  material  from  A  to  B,  12  ft.  wide,  and  that  even 
below  B  there  is  still  reef.  Even  in  this  case,  according  to 
Rand  practice,  the  face  would  be  divided  into  sections  of 
1 8  to  24  ins.,  and  each  would  be  sampled  and  assayed 
separately.  For  an  ordinary  drift  showing  unparted  reef, 
the  face  is  divided  into  three  sections,  namely,  "hanging 
wall,"  "middle"  and  "foot-wall,"  and  each  section  is 
sampled  separately. 

Thousands  of  tons  of  material  on  the  main  reef,  which 
is  the  bulky  producer  of  the  banket  series,  are  being  left 
in  the  mines.  As  a  rule,  this  material  is  too  poor  to  be 
worked  under  present  industrial  conditions,  but  there  are 
payable  patches  in  it,  and  in  the  future  more  attention 
will  be  paid  to  the  sampling  of  this  material.  An  ideal 
way  to  test  it  is  to  use  a  diamond  drill  and  get  cores  at 
regular  intervals  from  the  stuff  which  is  left  below  the 
foot  wall.  In  fact,  it  is  likely  that  the  diamond  drill  will 
become  an  important  auxiliary  in  mine  sampling. 

—134— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

In  sampling  drifts,  winzes  and  raises  it  is  found  of  con- 
siderable advantage  for  the  sampler  to  make  use  of  the 
surveyor's  mine  stations,  for  by  so  doing  the  exact  posi- 
tion of  each  sample  in  the  mine  is  determined. 

On  some  parts  of  the  Rand,  where  the  reef  is  only  an 
inch  thick,  it  is  a  common  practice  to  measure  off  equal 
distances  on  both  top  and  bottom  of  the  reef  and  take  the 
sample  over  this  width.  If  proper  care  is  taken  to  get  the 
right  proportion  of  country  rock  in  the  sample,  this 
method  is  a  good  one  for  small  rich  leaders.  Of  course, 
the  calculation  is  made  for  a  width  embracing  the  waste 
rock  sampled,  together  with  the  reef. 

Some  years  ago  panning  was  very  popular  on  the  Rand, 
but  it  has  gone  out  of  vogue  of  late.  On  account  of  the 
pyritic  character  of  the  matrix  and  the  fact  that  much  of 
the  gold  is  "bound,"  panning  can  never  be  relied  on  for 
accurate  determination  on  these  fields,  but  as  a  rough 
test  it  is  excellent.  Panning  serves  three  purposes :  First, 
in  a  large  mine  that  is  being  developed,  where  hundreds 
of  samples  are  assayed  each  month,  it  is  a  check  on  the 
assayer's  work.  Then,  panning  gives  a  close  approxima- 
tion of  the  amount  of  pyrite  in  the  sample,  so  that  the 
assayer  can  decide  whether  there  is  need  for  a  preliminary 
treatment  for  sulphur,  such  as  roasting,  etc.  Lastly,  in  an 
office  where  hundreds  of  samples  are  being  assayed  in  the 
course  of  mine  valuations,  I  believe  it  would  save  money 
if  the  samples  were  divided  according  to  the  results  from 
panning  into  rich  and  poor.  The  samples  high  in  gold 
could  be  assayed  by  the  scorification  method,  while  the 
crucible  would  be  used  for  the  poor  ones.  I  know  some 
peoply  decry  the  scorification  assay  and  demand  a  "pot 
assay"  for  every  sample,  but  with  proper  care  I  find  the 
scorification  assay  satisfactory  for  rich  ore. 

The  grouping  together  of  all  the  assay  values  is  en- 
trusted to  the  surveyor,  who,  of  course,  is  also  responsible 
for  the  determination  of  the  ore  reserves.  The  assay  plan 
contains  the  values  of  the  samples  taken  in  the  mine,  and 
it  is  this  plan  that  the  surveyor  uses  in  estimating  the 
reserves.    Assay  plans  are  made  on  one  of  three  different 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

planes,  namely,  the  horizontal,  the  vertical,  and  the  plane 
of  the  reef ;  the  one  which  is  best  adapted  for  the  purpose 
will  depend  upon  the  dip  of  the  reef. 

It  is  when  the  mill  starts  crushing  and  gold  returns  are 
reported  that  the  mine  valuation  is  finally  tested.  I  must 
admit  that  it  is  somewhat  discouraging  to  review  the  past 
history  of  the  Rand  and  notice  the  difference  between 
what  was  promised  before  the  mill  started  and  what  was 
actually  obtained  when  the  battery  commenced  to  work. 
If  there  was  a  constant  in  the  discrepancy,  it  would  be  a 
simple  matter  to  apply  it  to  future  determinations,  but  the 
results  are  too  contradictory. 

On  one  mine  it  was  found  that  the  returns  from  the 
reduction  works  were  70  per  cent,  of  what  the  assay  plan 
called  for.  On  a  neighboring  mine  this  factor  was  subse- 
quently employed.  When  the  mill  commenced  crushing, 
however,  much  to  the  delight  of  every  one  it  was  found 
that  a  pennyweight  more  gold  per  ton  was  extracted  than 
was  promised  by  the  estimates. 

This  goes  to  prove  that  the  value  obtained  after  sam- 
pling is  not  a  mathematically  correct  figure,  and  that  an 
engineer  cannot  always  guarantee  that  the  mine  will  give 
what  the  sampling  calls  for.  In  spite  of  such  occasional 
discouraging  results,  sampling  on  the  Rand  is  looked  upon 
with  more  favor  than  ever,  and  with  extended  local  experi- 
ence the  work  of  estimation  is  certain  to  be  better  done. 

From  what  has  been  said,  it  must  not  be  thought  that 
the  work  of  the  sampler  is  over,  or  lessened,  when  the 
mill  commences  crushing.  At  all  stages  the  sampler  is 
necessary.  After  mining  begins  he  watches  the  stopes 
most  carefully  to  see  that  no  payable  material  is  left  in 
the  workings.  From  the  stope  samples,  taken  throughout 
the  month,  the  value  per  ton  of  what  the  mill  should  re- 
turn for  that  month  is  worked  out.  Every  day  a  '*lip 
sample"  is  also  sent  up  from  the  mill.  As  inside  amalga- 
mation is  now  almost  entirely  omitted,  only  a  spoonful  of 
mercury  being  fed  into  the  boxes  every  hour,  with  proper 
care  a  very  accurate  sample  of  the  ore  being  crushed  is 
given  by  the  *1ip  sample." 

—136- 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

I  am  glad  to  see  that  Mr.  Rickard  condemns  the  "grab" 
sample  so  emphatically.  Better  not  sample  at  all  than 
adopt  this  haphazard  method.  The  only  place  where  the 
"grab"  sample  is  employed  is  in  testing  the  waste  rock 
leaving  the  sorting  house. 

Space  will  not  allow  me  to  enter  into  the  discussion  of 
ore  reserves.  The  expression,  "ore  in  sight,"  which  ap- 
pears to  be  causing  so  much  discussion  among  American 
engineers,  is  seldom  used  out  here  at  all.  "Tons  of  ore 
developed"  is  employed,  and  care  is  taken  to  give  the 
number  of  tons  developed  on  each  separate  reef.  As  Mr. 
Philip  Argall,  in  his  suggestive  contribution,  has  pointed 
out,  the  engineer  should  go  into  detail  and  give  a  precise 
statement  as  to  the  condition  of  the  "ore  in  reserve."  Al- 
most invariably  on  the  Rand  the  number  of  tons  of  ore  in 
reserve  are  underestimated,  due  to  too  small  a  stoping 
width  being  assumed  in  the  calculation.  The  question  of 
stoping  width  is  a  very  important  one,  for,  if  it  is  wrongly 
assumed,  not  only  is  the  tonnage  too  small,  but  the  assay 
value  over  the  stoping  width  is  made  too  high.  At  one 
time  the  stoping  width  was  arrived  at  in  the  office,  but 


now  the  custom  is  to  examine  every  block  of  ground  and 
work  out  carefully  what  the  actual  stoping  width  will  be. 
On  those  mines  which  have  the  misfortune  to  be  cut  up 


137- 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

by  dikes  the  management  is  at  considerable  disadvantage 
as  regards  ore  development.  Fig.  3  will  illustrate  a  case. 
The  dike  cuts  in  two  the  block  of  ground  developed  by  the 
fourth  and  fifth  levels.  Ordinarily,  the  calculation  for  re- 
serves would  be  made  for  backs  from  A  to  D.  The  general 
Rand  practice  is  to  only  consider  the  backs  from  A  to  B  in 
working  out  reserves.  From  C  to  D  is  not  considered 
developing  ground  at  all. 

On  most  mines  the  amount  of  ground  developed  each 
month  is  usually  at  least  as  much  as  the  tonnage  mined 
during  that  month.  Unfortunately  just  now,  on  account 
of  the  scarcity  of  labor,  it  is  found  difficult  to  maintain  a 
parity. 

T.  Lane  Carter. 

Johannesburg,  April  4,  1903. 


SOME  ASPECTS  OF  MINE  VALUATION. 

The  Editor : 

Sir. — As  there  are  no  gold  mines  in  Scotland,  I,  a 
would-be  valuer  of  mines,  had  to  seek  my  ideals  elsewhere. 
I  found  them  in  American  methods.  The  American  engi- 
neer goes  at  once  to  the  vital  point.  He  says :  '7  want  to 
know  the  net  profit  in  sight  in  this  mine.  On  that  one  fact 
we  will  base  everything.  Don't  bother  me  with  details; 
just  let  us  set  to  work  to  get  the  sampling  through."  At 
least,  if  he  doesn't  say  that  he  ought  to,  because  it  is  the 
one  thing  in  mining  that  matters.  Frankly,  I  have  got 
"profit  in  sight"  on  the  brain. 

There  is  no  marked  cleavage  between  the  American  and 
the  English  schools  as  regards  mine  valuation;  but  an 
Englishman,  as  a  rule,  has  got  a  less  clearly  defined  idea 
of  the  net  value  of  any  given  mine  than  an  American. 
As  a  mine  manager  he  is  perhaps  the  better  man  of  the 
two,  but  on  the  question  of  valuation  he  is  not  so  clear. 
There  is  a  certain  haziness  and  slovenliness  about  English 
gold  mining.  Some  of  the  mines  use  a  ton  of  2,000  lbs. ; 
others,  one  of  2,240  lbs.  We  calculate  our  yields  and  our 
assays  in  the  wearisome  Troy  weight,  instead  of  in  that 

—1 38— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

Sterling  with  which  nature  has  supplied  us.  We  put  our 
best  properties  under  the  control  of  titled  incompetents, 
and  we  write  reports  on  mines  so  lacking  in  a  grasp  of  the 
vital  points  that  the  compilers  ought  to  be  led  out  at  sun- 
rise and  shot. 

This  slovenliness  in  the  English  system  shows  itself, 
naturally,  in  the  all-important  branch  of  mine  valuation. 
We  do  not,  it  seems  to  me,  rush  in  and  grasp  the  one  great 
fact — the  profit  in  sight — as  do  Americans.  We  play 
round  the  edges  of  it,  and  our  reports,  which  should  be 
merely  a  statement  of  this  fact  in  its  several  bearings,  are 
made  bulky  with  useless  details  and  side  issues.  I  believe 
that  often,  too,  having  got  the  fact,  we  don't  quite  know 
how  to  use  it.  We  cannot  crystallize  it  sufficiently  into 
thought.  If  our  fact,  as  is  so  often  the  case,  shows  the 
mine  to  be  on  the  borderland  of  good  and  bad,  in  the  re- 
gion of  doubt,  we  are  afraid  to  say  to.  our  principal, 
definitely,  either  "yes"  or  "no."  We  try  to  shift  the  re- 
sponsibility of  the  decision  to  someone  else,  and  write  a 
report  which  has  no  backbone  or  individuality  about  it.  I 
think  it  takes  a  strong  man  to  be  a  good  mine  valuer — a 
man  to  whom  the  taking  on  of  responsibility  comes  natu- 
rally. You  must  know  how  to  handle  your  fact  to  be  suc- 
cessful. An  able  engineer  told  me  that  he  judged  his 
subordinates  by  the  way  they  could  condense  a  report  into 
the  limits  of  a  cable ;  and  the  shortest  cable  meant  the  best 
man.  I  think  he  was  not  far  wrong.  He  was  an 
American. 

English  engineers  often  fall  short  of  doing  good  work 
because  they  are  unable  to  view  the  problem  from  their 
employer's  point  of  view.  They  get  their  fact  all  right — 
that  is  to  say,  the  net  value  of  the  mine — but  they  don't 
know  how  that  fact  ought  to  be  set  before  the  capitalist. 
To  be  a  good  mine  valuer,  a  man  must  have  the  instincts 
of  a  financier.  He  must  put  himself  in  that  particular 
financier's  place.  He  would  then  realize  how  needful  it  is 
that  his  report  should  be  a  clear-cut  statement,  saying  yes 
or  no  to  each  question  and  taking  full  responsibility.  As 
it  is,  four  out  of  five  reports  I  read  are  written  by  men  who 

—139— 


SA.MPLINCr    AND    ESTIMATION    OF    ORE, IN    A    MINE 

hedge,  who  are  weak  on  the  financial  aspect,  and  who 
leave  an  impression  of  vagueness  that  is  exasperating. 
Tliese  same  reports  will  be  voluminous,  and  deal  at  length 
with  side  issues,  but  they  don't  satisfy  one  on  the  vital 
points.     The  system  is  wrong. 

The  environment  of  the  English  engineer  is  against  him. 
In  America  the  capitalist  who  sends  a  man  to  value  a  mine 
probably  intends  to  buy  that  mine  himself  and  work  it ; 
and  as  likely  as  not  he  knows  nearly  as  much  about  mining 
as  the  engineer  does.  His  first  question  to  the  mine  valuer 
on  his  return  is,  ''Well,  what's  the  net  profit  in  sight  ?"  He 
takes  the  report  and  turns  it  inside  out,  and  if  in  his  sev- 
eral capacities — as  mining  man,  financier  and  student  of 
men — it  fails  to  satisfy  him,  its  compiler  may  understand 
that  he  has  been  "turned  down."  But  the  London  capital- 
ist is  a  man  who  does  not  buy  mines  to  work  himself.  He 
is  essentially  a  middleman.  His  life's  work  consists  in 
educating  a  large  public  to  come  forward  and  buy  the 
wares — good  or  bad — which  he  may  from  time  to  time 
offer  them.  He  would  hardly  appreciate  the  significance 
of  being  told  that  he  had  got  hold  of  a  valuable  mine.  To 
him  the  great  question  would  be,  "Can  I  get  the  shares 
well  introduced  on  the  market  ?"  On  your  returning  from 
the  mine,  his  first  question  would  be,  not  "What  is  the  net 
value  of  the  ore?"  but,  "Can  you  lunch  here  to-morrow? 
I  want  you  to  explain  your  report  to  Lord  So-and-So,  who 
is  to  be  our  chairman." 

The  English  school  of  mining  is  seen  at  its  worst  in 
Australia :  that  is  to  say,  in  Australian  mines  controlled 
and  managed  locally.  Some  of  these  mines — for  instance, 
those  at  Gympie,  Queensland — are  pockety,  and  would  be 
difficult  to  sample  correctly,  but  four  out  of  any  five  mines 
in  Australia  might  be  sampled  with  some  exactness.  The 
Australian,  however,  flatly  refuses  to  sample  any  of  his 
mines ;  he  just  goes  along  from  one  ore-shoot  to  another, 
picking  the  eyes  out  of  each,  and  so  far  as  this  great  min- 
ing continent  is  concerned — with  the  exception  of  West 
Australia,  whose  mines  are  owned  in  England — mine 
sampling,  or  valuation,  is  almost  an  unknown  art.    In  the 

—140— 


SAMPLINO    AND    ESTIMATION    OF    ORE    IN    A    MINE 

Melbourne  Argus  on  a  Monday  morning  there  will  be  the 
weekly  reports  of  perhaps  300  local  gold  mines.  In  these 
the  managers  will  describe  development  work  as  "exposed 
gold,"  "good  gold,"  ''pyrites,"  "colors,"  "pay  dirt,"  etc., 
etc.,  but  from  beginning  to  end  there  will  be  no  word  of 
either  systematic  sampling,  assay  results,  or  estimates  of 
ore  reserves. 

Now,  mark  the  force  of  evil  example.  The  gold  mining 
industry  in  the  Malay  Peninsula  was  started  by  Australian 
prospectors,  and  they  were  succeeded  by  Australian  mine 
managers  trained  to  these  methods.  The  principal  mine 
there  was  the  Raub.  Some  years  ago  I  was  asked  to  re- 
port on  it,  because  of  its  failing  to  keep  up  to  previous 
yields.  The  mine  had  been  working  for  eleven  years,  and 
at  that  time  had  a  60-stamp  mill.  The  manager  had  just 
written  a  report  stating  that  there  were  300,000  tons  of 
rich  ore  in  sight  (value,  about  $20),  and  that  the  lowest 
workings  showed  no  falling  off  in  results. 

Then  I  made  the  following  discoveries :  First,  that  the 
mine  had  never  been  sampled;  second,  that  there  were  only 
41,000  tons  of  pay-ore  reserves  (value,  $9)  ;  lastly,  that 
the  lowest  workings  in  the  mine  showed  no  pay-ore.  This 
property  at  the  time  was  valued  at  $6,000,000. 

That  was  an  extreme  case — even  under  the  Australian 
system — ^but  it  is  a  warning  that  careless  mine  managers 
might  take  to  heart. 

I  started  to  write  this  article  under  the  idea  that  I  was 
about  to  make  some  profound  remarks  on  the  minutiae 
of  mine  valuation — about  sampling  methods,  irregular 
values,  ore  reserves,  and  so  forth.  But  after  reading  Mr. 
Rickard  on  "Mine  Sampling,"  Mr.  Argall  on  "Ore  Re- 
serves," and  other  authorities  who  have  lately  spoken  in 
this  paper,  my  assurance  is  not  what  it  was.  They  have 
left  little  loophole  for  any  one  coming  after. 

I  have  never  allowed  a  sample  to  be  broken  down,  ex- 
cept in  my  presence.  However  much  T  trusted  an  assist- 
ant, I  could  only  accept  the  assurance  of  my  eyes  that  the 
section  in  question  was  fairly  represented  by  the  10,  20  or 
50  pounds  of  ore  going  into  the  bag.     Soft  streaks  in 

—141— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

the  ore  generally  mean  richness — hard  streaks,  poverty. 
The  adjustment  of  the  two  is  a  problem  which  only  one's 
own  conscience  can  satisfy. 

I  have  my  assays  done  in  duplicate,  especially  so  when 
there  is  a  plant  on  the  mine  and  I  can  take  my  own  assayer. 
If  the  beads  vary  only  slightly  in  weight,  you  cannot  but 
feel  the  more  satisfied  with  the  eventual  result.  At  the 
Chuquitambo  mine,  in  Peru,  a  conglomerate  ore  assaying 
$5,  in  four  samples  out  of  five,  my  duplicate  beads  used 
almost  to  balance.  For  instance,  a  series  would  read: 
4.01,  4.02 — 6.83,  6.85 — 2.31,  2.31 — ^4.98,  5.00.  When 
there  is  a  big  diflFerence  it  is  as  well  to  re-assay  the  sample 
— not  to  take  the  mean  of  the  two  beads.  I  expect  to  be 
sampling  a  mine  in  Hungary  shortly,  where  the  ore  carries 
tellurides ;  there  I  shall  have  the  assays  done  in  triplicate. 

The  question  of  high  assays  is  a  difficult  one.  Even  Mr. 
Rickard  handled  this  question  gingerly.  As  he  implied,  it 
is  one  which  varies  with  nearly  every  mine ;  it  cannot  be 
reduced  to  definite  system.  In  some  mines — but  I  think 
very  few — the  occasional  high  assay  truly  represents  the 
nature  of  the  ore.  Mr.  Rickard's  example  of  the  Tomboy 
mine,  as  falling  under  this  heading,  is  also  the  one  I 
would  suggest.  If  the  fantastic  assays  at  Tomboy  were 
eliminated,  the  ore  would  work  out  at  an  average  recovery 
of  $4  or  $5 — ^but  it  is  actually  double  that.  At  the  adja- 
cent Camp  Bird  mine,  the  assays  of  which  run  into  hun- 
dreds of  dollars — and  they  occur  rather  often — are  judged 
in-  their  relation  to  the  assays  on  each  side,  and,  should 
those  be  much  lower,  are  reduced  considerably.  For  ex- 
ample, three  following  assays  going  $52 — $165 — $79 
would  be  entered  on  to  the  plan,  but  in  a  series  reading 
$5 — $104 — $11,  the  middle  assay  would  be  drastically  cut 
down.  In  the  Camp  Bird  there  are  so  many  very  high 
assays  that  this  method  leaves  the  management  with  a 
something  "up  its  sleeve,"  but,  for  accuracy,  it  is  nearer 
the  mark  than  putting  every  high  sample  on  to  the  plan  at 
its  theoretical  value.  On  the  Rand,  at  least  in  my  day, 
exceptional  assays  were  rigidly  reduced  to  something  like 
the  general  average  of  the  mine.     I  feel  sure  this  was 


SAMPLINC;    AND    ESTI.MA HON    OF    ORE    IX    A    MINE 

correct.  The  collapse  of  many  of  the  outside  Transvaal 
mines,  banket  and  quartz,  may  be  traced  to  the  fact  that 
their  sampling  was  inefficient,  and  that  their  incapable 
managers  had  included  all  high  assays  at  their  full  value. 
Mr.  Denny  has  written  very  well  on  the  subject  of  sam- 
pling on  the  Rand.  This  year  I  sampled  a  certain  mine. 
I  took  seventy-five  samples,  of  which  four  gave  fantastic 
results,  showing  coarse  gold.  The  general  average  with- 
out these  was  unpayable,  so  I  was  spared  the  trouble  of 
even  re-sampling  at  these  spots.  I  included  these  four 
samples  on  the  assay  plan,  giving  to  each  a  value  four 
times  greater  than  the  general  average  without  them,  and 
thereby  raising  the  general  average  $i  a  ton.  The  result, 
curiously  enough,  brought  the  theoretical  value  of  the  ore 
out  at  the  precise  figure  which  some  thousands  of  tons 
had  already  yielded  in  actual  treatment. 

As  Mr.  Rickard  pointed  out,  you  may  either  re-sample 
a  rich  spot  or  you  may  sample  on  each  side  of  it,  close  up, 
and  take  the  mean  of  the  two.  My  own  idea  is  that  in  a 
low-grade  ore,  especially  one  carrying  regular  values,  a 
high  assay  is  out  of  place.  Even  if  I  re-sampled  such  a 
spot  a  dozen  times,  and  kept  getting  higher  and  higher 
results,  I  should  still  cut  it  down  to  the  average.  In  a 
high-grade  ore  it  would  be  more  natural  to  find  a  few  big 
assays,  but  I  would  not  recommend  the  purchase  of  a  mine 
on  them — the  margin  would  have  to  be  represented  with- 
out their  assistance. 

In  figuring  out  ore  reserves  a  mine  valuer  has  got  to 
take  certain  risks,  for  not  one  mine  in  fifty  changes  hands 
on  the  basis  of  net  profit  in  sight.  If  the  ore-shoot  under 
valuation  is  of  biggish  dimensions  and  the  lode  well  de- 
fined, I  think  the  engineer  is  entitled  to  allow  something 
for  "good-will."  In  such  a  case,  I  should  allow  25  or  30 
ft.  beyond  the  face  of  a  drive  in  good  ore ;  below  a  single 
winze  I  would  allow  10  ft.  and  below  two  winzes  not  a 
great  distance  apart,  25  ft.  A  similar  margin  of  risk^ 
would  hold  good  at  other  development  points.  If  the  ore- 
shoot  was  a  short  one,  or  a  series  of  short  shoots,  and  the 
lode  of  "scraggy"  appearance,  I  do  not  think  T  would 

—143— 


SAMPLING    AND    ESTIMATION    OF   ORE  >IN    A    MINE 

make  any  allowance  at  all.  In  either  case,  one  would  be 
guided  by  the  condition  of  neighboring  mines. 

Of  course,  it  were  better  to  be  in  the  position  of  never 
taking  a  risk  beyond,  let  us  say,  a  block  of  ore  exposed  on 
two  sides — but,  as  I  have  sa^d,  mines  can  rarely  be  bought 
on  such  conditions.  The  mine  valuer  knows  that  he  is 
liable  to  error,  and  that  the  ore  "assumed  to  exist"  may 
turn  out  valueless.  But  if  he  is  an  honorable  man,  whose 
experience  tells  him  he  is  justified  in  taking  such  risks, 
I  see  nothing  wrong  with  the  system.  If  a  man  is  true  to 
himself,  he  is  not  dealing  falsely  by  his  employers. 

With  the  assistance  of  a  good  surveyor  and  draughts- 
man and  of  a  reliable  assayer,  there  is  no  reason  why  a 
mine  valuer  who  is  very  careful,  who  is  practical,  who  is 
experienced,  and  who,  down  in  the  depth  of  his  heart,  has 
confidence  in  himself,  should  not  arrive  at  a  fairly  cor- 
rect estimate  of  the  intrinsic  value  of  most  ore-bodies. 
But  he  must  be  practical.  In  mine  valuation,  practice 
and  theory  are  apt  to  clash  rudely.  I  once  sampled  a  gold 
mine  which  had  previously  been  reported  on  by  one  who 
wrote  after  his  name  "Lecturer  on  Mining."  He  had  fine 
credentials.  This  man,  as  a  theorist,  was  in  the  upper 
ranks  of  mining,  but  in  practice,  as  a  valuer  of  mines,  I 
found  him,  to  say  the  least,  eccentric.  He  satisfied  him- 
self with  only  thirteen  samples  altogether,  although  the 
workings  were  quite  extensive;  and  from  the  results  of 
these  assumed  the  average  value  of  60,000  tons  of  ore. 
One  block  of  18,000  tons  he  valued  on  a  single  sample. 
Across  this  same  spot  my  own  sample  gave  $30  less  than 
his,  and  the  rest  of  the  block,  from  samples  taken  every 
few  feet,  was  still  poorer.  My  valuation  for  the  whole 
mine,  from  a  big  number  of  assays,  worked  out  at  one- 
third  of  his,  and  the  tonnage  at  one-third.  I  do  not  infer 
that  my  own  figures  were  necessarily  correct;  but  those 
who  style  themselves  "Lecturers  on  Mining"  should  real- 
ize that  mines  cannot  be  summarized  in  thirteen  samples, 
nor  1 8,000-ton  blocks  of  ore  by  single  assays.  This  per- 
son, I  believe,  meant  well  by  his  report,  but  nature  did  not 
fit  him  with  that  practical  and  active  nature  necessary  to  a 

—  144— 


SAMPLING,  AND    ESTIMATION    OF    ORE   IN    A   MINE 

valuer  of  mines.     As  a  plumber  he  might  have  been  a 
success. 

J.    H.    CURLE. 

London,  April  25,  1903. 


The  Editor : 

Sir. — The  interesting  series  of  articles  recently  pub- 
lished in  the  Journal  upon  mine  sampling  suggest  com- 
ments resulting  from  the  writer's  own  experience. 

In  the  first  place,  it  seems  that  the  prospect  pick  should 
not  be  so  summarily  cast  aside.  In  cases  where  the  engi- 
neer is  sampling  small  prospects,  or  must  get  along  with- 
out an  assistant  other  than  a  local  helper  whose  sympa- 
thies are  with  the  mine,  it  is  possible  with  a  proper  use 
of  the  head  and  the  point  to  get  a  fair  sample.  In  the 
taking  of  check  samples  and  for  testing  particular  ore 
streaks  it  is  very  useful,  if  its  weak  features  are  borne 
in  mind.  The  use  of  *'pop  shots"  is  admittedly  unwise, 
but  the  flat  cone  resulting  from  their  use  is  no  more  unfair 
than  the  numerous  irregularities  of  face  encountered  in 
sampling.  The  salting  of  the  dynamite  is  all  but  impos- 
sible, if  the  sticks  are  taken  out  of  the  original  package 
and  the  miner  has  not  been  warned.  Another  point 
worthy  of  attention  is  the  condition  of  the  face  to  be  sam- 
pled. This  is  very  often  dirty  with  mud,  powder  smoke, 
and  dust.  Aside  from  the  actual  danger  of  salting,  always 
possible  with  dirty  faces,  it  must  be  remembered  that  the 
rich  ore  minerals  of  many  deposits  are  brittle  and  the  fines 
are  very  rich.  Such  material  may  give  false  values  to  a 
sample  taken  from  a  dirty  face.  If  the  ore  be  very  hard, 
the  only  way  to  get  a  clean  face  may  be  to  use  pop  shots. 

A  word  about  the  use  of  a  box  to  catch  the  ore  cut  in 
sampling.  If  the  ore  be  very  hard  it  will  fly  off  in  chips, 
a  considerable  part  of  which  will  not  go  into  the  box ;  if 
soft  streaks  occur,  all  the  material  goes  into  the  box.  The 
result  is  an  unfait  sample.  If  a  strip  of  canvas  20  ft. 
long  be  used  and  the  width  is  sufficient  to  reach  the  sides 
of  the  drift,  all  the  chips  will  be  caught.     There  will  be  no 

—145— 


SAMPLING    AND    ESTIMATION    OF    ORE    IX    A    MINE 

danger  of  any  one  salting  the  sample  by  flipping  in  ma- 
tcrial,  if  the  engineer  follows  the  commendable  practice 
of  excluding  every  one  from  the  workings  but  his  own 
men.  The  use  of  the  canvas  is  also  an  advantage  if  the 
<^re  be  spotty,  since  it  is  easy  to  cut  a  number  of  narrow 
grooves  at  close  intervals,  letting  the  material  all  form  one 
sample. 

In  quartering  down  the  sample  it  is  not  always  possible 
or  convenient  to  use  the  method  described  by  Mr.  Rickard, 
especially  in  the  examination  of  small  and  remote  prop- 
erties. If  the  usual  method  of  rolling  the  sample  on  can- 
vas be  used,  the  fines  will  be  more  uniformly  distributed 
if  the  canvas  be  rolled  by  one  person,  taking  hold  of  oppo- 
site corners  alternately  and  rolling  with  the  cloth  close 
against  the  sample.  Too  often  caution  is  not  used  to 
brush  off  the  dust  when  the  rejected  quarters  are  removed. 
Personally,  I  find  a  piece  of  white  enameled  table  oil-cloth 
preferable  to  canvas  in  quartering  down.  It  is  quite  as 
flexible  and  the  glazed  surface  prevents  the  material  from 
sticking,  while  the  cloth  can  be  bought  at  any  country 
store. 

In  marking  samples  some  engineers  carry  small. brass 
disks  with  numbers  punched  on  them.  If  used  at  hap- 
hazard, and  the  number  entered  in  the  notebook,  identi- 
fication by  other  parties  is  impossible.  If  numbers  are 
put  on  the  outside  of  the  sacks,  do  not  make  them  con- 
secutive. 

In  extreme  cases,  as  in  long  trips  in  Mexico,  where  the 
samples  cannot  be  under  observation  all  the  time,  it  is 
safer  to  use  tin  cans  made  and  taken  out  for  this  purpose 
and  sealed  after  filling. 

When  inspecting  a  mine  it  is  both  good  sense  and 
economy  to  make  a  preliminary  examination,  and  to  take 
samples  at  long  intervals.  Personally,  I  believe  it  is  good 
practice  to  take  samples  of  foot  and  hanging-wall  streaks, 
even  where  apparently  barren,  also  of  separate  ore-streaks 
and  lenses.  It  affords  valuable  information  about  the  dis- 
tribution of  values  and  as  to  the  advisability  of  sorting  the 
ore.     In  my  own  experience  I  have  found  the  waste  of 

— 146 — 


SAMPLING    AND    ESTIMATION    OK    ORE    IN    A    MINE 

altered  gneiss,  constituting  the  vein-filling  alongside  of  a 
vein  of  galena,  to  be  full  of  cobweb-like  films  of  rich  silver 
ore  (pearcite),  constituting  better  ore  than  the  galena,  and 
yet  this  had  been  thrown  over  the  dump  for  some  time. 
The  discovery  of  the  Camp  Bird  by  Mr.  Walsh  was,  he 
tells  me,  due  to  his  recognition  of  the  value  of  a  similar 
condition  but  of  different  character.  In  another  mine  I 
found  the  hanging  was  an  altered  andesite  containing 
pyrite  and  carrying  $60  per  ton  in  gold,  while  the  recog- 
nized pay-ore  carried  only  $12  per  ton.  While  it  is  true 
that  the  local  miner  generally  knows  the  ore  as  well  as,  or 
better  than,  the  examining  engineer,  it  is  well  not  to  fol- 
low the  miner  in  his  prejudices. 

The  valuation  of  a  block  of  ore  being  based  on  assay 
results,  it  is  well  to  consider  how  these  results  should  be 
used.  The  common  method  is  to  multiply  each  assay 
value  by  the  width  of  ore  at  the  point  where  the  sample 
was  taken,  adding  up  the  resulting  "foot-dollars"  and 
striking  an  average,  to  be  used  with  the  tonnage  of  each 
block  to  give  the  total  value.  In  still  more  accurate  work, 
two  assays  are  made  of  each  sample,  and  these  are  aver- 
aged. Another  way,  and  one  which  has  given  very  re- 
liable results,  is  to  average  the  values  of  two  adjacent 
samples  and  multiply  the  result  by  the  average  of  the  two 
widths  of  the  vein ;  the  product  is  to  be  used  as  the  value 
of  that  particular  part  of  the  ore  block ;  the  result  is  quite 
different  from  that  obtained  by  the  first  method,  as  will 
be  seen  below : 

Width....  2  (6)  10  (6.5)  3  (2)  I  (5.5)  TO 
Value    $20     ($11)     $2     ($10)     $18     ($14)     $10     ($20)     $30 

The  average  is  $16.31,  using  the  first  method,  and 
$13.45,  using  the  last. 

Dressing  a  mine  for  examination  is  easier  than  salting 
and  more  difficult  to  detect.  If  the  ore  occurs  in  bunches 
or  small  shoots  it  is  an  easy  matter  to  carry  up  a  stope 
as  long  as  the  ore  is  low  grade,  or  the  material  is  waste, 
and  to  stop  work  as  soon  as  good  ore  is  reached.  In  a  cer- 
tain Wood  River  property  the  mine  was  so  well  dressed 
that  a  reliable  engineer  figured  that  the  ore  developed  was 

—147— 


SAMPLING    AND    ESTIMATION    OF    ORE    UST    A    MINE 

worth  $450,000,  and  when  it  was  worked  it  was  fotind  to 
be  but  $125,000.  In  a  recent  case  a  milHon-dollar  prop- 
erty, with,  what  appeared  to  be,  a  safe  margin  of  profit  in 
the  great  blocks  of  ore  developed,  was  shown  by  the  work 
after  purchase  to  have  been  similarly  dressed  for  examina- 
tion. The  irregular  workings  of  some  mines  and  a  lack 
of  rectilinear  development  are  likewise  misleading.  Where 
the  workings  follow  irregular  ore-streaks  an  ordinary 
calculation  is  apt  to  give  a  wholly  erroneous  estimate  of 
the  ore  developed.  This  is  particularly  true  of  Mexican 
mines,  where  the  irregular  workings  follow  the  ore  in 
utter  disregard  of  any  other  consideration,  such  as  econ- 
omy of  extraction  or  systematic  development. 

Walter  Harvey  Weed. 
Washington,  July  7,  1903. 


The  Editor: 

Sir. — Referring  to  Mr.  Weed's  letter  of  July  7,  it  seems 
desirable  to  call  attention  to  the  fact  that  his  second  meth- 
od of  obtaining  averages  is  indefensible.  Starting  at  the 
root  of  the  matter,  the  true  average  of  any  set  of  samples  is 
obtained  by  multiplying  the  value  of  each  by  the  width  of 
each  and  dividing  the  sum  of  the  products  by  the  sum  of 
the  widths.  The  average  width,  of  course,  in  the  case  where 
samples  are  taken  equi-distant  is  the  numerical  average  of 
the  widths.  This  has  been  explained  recently  in  the  series 
of  articles  in  the  Journal  that  prompted  this  discussion ; 
a  little  earlier  Mr.  A.  G.  Charleton  referred  to  the  matter 
in  Volume  IX.  of  the  Transactions  of  the  Institution  of 
Mining  and  Metallurgy,  and  once  at  least  in  an  earlier 
volume  of  our  own  Institute  the  subject  has  been  dis- 
cussed. All  of  these  papers  have  been  of  admirable  clear- 
ness, and  will  certainly  bear  re-reading  at  intervals  by 
every  engineer.  In  passing  on,  I  would  like  to  call  atten- 
tion to  one  detail :  While  the  average  value  of  the  rock 
will  truly  be  determined  as  above,  the  average  stoping 
value  of  the  rock  in  cases  where  the  widths  run  down  to  a 

—148— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

Space  smaller  than  that  in  which  a  man  can  work  must, 
for  correctness,  be  referred  to  least  stoping  width. 

However,  this  is  a  matter  aside.  Returning  to  the 
question  in  hand,  I  append  below  a  list  showing  the  aver- 
age values  of  the  five  samples  according  to  Mr.  Weed's 
two  methods,  substituting  for  his  results  those  obtained 
by  using  the  proper  average  width  divisor,  and  the  aver- 
ages also  worked  out  (rather  as  matter  of  curiosity)  for 
the  second  method,  but  using  true  averages  for  the  sections 
between  each  pair  of  samples.  As  a  further  explanation  of 
this  last  let  it  be  noted  that  Mr.  Weed's  parenthetical  val- 
ues in  nowise  represent  the  true  values  for  those  sections, 
as  they  are  bare  numerical  averages,  giving  the  same 
weight  to  a  two-foot  sample  and  a  ten-foot  sample.  Thus 
he  gives  the  average  for  the  width  between  the  first  two 
samples  as  6  ft.,  which  is  correct ;  he  obtains  his  average 
value  in  the  same  way : 

^^°  "^  ^^  equals  i^ii.oo.    The  error  is  evident: 

2 

;5520.oo  X    2  ft.  equals  40  foot-dollars. 
2.00  X  10  "         "     _20     "        " 
12 "        "      60    "        " 
60 


.2 


equals,  ^5,  the  true  average  for  the  first  section. 


But  there  is  another  error  that  crops  up  in  this  method 
of  sections  or  panels ;  it  is  that  the  three  middle  samples 
are  used  twice,  the  first  and  last  but  once ;  and  as,  in  this 
case,  the  first  and  last  samples  are  the  highest,  the  re- 
sulting average  is  too  low.  Of  course  with  a  large  suite 
of  samples,  such  as  would  obtain  in  most  practical  work, 
this  error  would  not  be  large,  but  it  is  hard  to  see  any  ex- 
cuse for  this  method  in  mine  valuation.  The  comparison 
is  as  follows : 
True  averages  as  given  by  Mr.  Weed's  first 

method $16.31  5.2  ft. 

Average  by  sections,   Mr.  Weed's   second 

method $1345  5-0  ft. 

The  same  with  true  sectional  averages $12.69  5-0  ft. 

This   comparison   satisfactorily   shows   the   extent   of 

—149— 


SAMPLING    AND    ESTIMATION    OF    ORE    fN    A    MINE 

error  easily  obtainable  by  an  incorrect  system  of  averag- 
ing, which  extends  to  width  as  well  as  value. 

All  of  us,  I  fancy,  have  had  experience  of  the  matter 
mentioned  in  ^Ir.  A\'eed's  last  paragraph,  of  a  mine  being 
"dressed  for  examination."  It  is  the  common  dodge  of 
the  prospector,  who  runs  well  into  a  good  "bunch,"  and 
leaves  it  for  the  engineer  to  sample  and  goes  to  another 
part  of  the  property  to  repeat  the  "dressing,"  But  there 
is  another  point,  which  calls  for  the  careful  investigation 
of  the  engineer :  It  is  a  common  claim  among  promoters 
that  the  actual  results  from  mill-runs  furnish  a  better 
indication  of  the  value  of  the  ore  reserves  in  a  property 
than  any  amount  of  sampling,  and  this  claim  can  be  so 
plausibly  supported  that,  by  the  investor  at  least,  it  is  fre- 
quently accepted  as  truth.  As  a  matter  of  fact  the  own- 
ers are  apt  to  be  pressed  for  money,  otherwise  they  might 
not  have  wished  to  sell  the  mine,  and  in  that  case  they  are 
sure  to  have  handled  the  better  grade  of  rock,  so  that  the 
result  of  past  operations  would  not  in  any  way  represent 
the  value  of  the  ore  left.  I  have  a  case  in  mind  where  the 
drifts  were  at  times  run  very  high.  The  high  spots  turned 
out  to  be  in  particularly  good  ore;  in  this  case  but  little 
stoping  had  been  done  and  considerable  development,  and 
until  one  had  seen  the  property  the  claim  that  the  value 
of  the  ore  extracted  represented  the  value  of  the  ore  left 
seemed  a  most  plausible  one. 

R.  Oilman  Brown. 

San  Francisco,  July  25,  1903. 


The  Editor: 

Sir. — In  common  with  many  of  your  readers  I  have  en- 
joyed the  discussion  that  has  appeared  in  your  columns 
over  the  methods  and  details  of  mine  sampling,  and,  in 
connection  therewith,  I  am  tempted  to  break  the  silence 
generally  held  by  me,  to  mention  an  interesting  incident 
of  sampling  that  occurred  in  a  mine  in  California. 

The  ore  was  the  usual  quartzose  schist  of  the  middle 
counties  of  the  Mother  Lode.       About  50  lb.  had  been 

—ISO— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

broken  from  a  12-ft.  face  of  ore,  and  while  wet  and  a  little 
sticky,  it  was  broken  down  to  about  3^-in.  pieces;  after 
the  usual  rolling  and  quartering  upon  ordinary  cotton 
duck,  it  occurred  to  me  that  it  would  be  interesting  to  take 
the  two  final  quarters,  crushed  to  about  ^s  in.,  and  weigh- 
ing about  10  lb.,  as  two  samples,  and  have  them  assayed 
separately,  because  it  is  generally  assumed  that  the  final 
quarterings  are  of  equal  assay  value. 

Much  to  my  surprise  one  quarter  assayed  $14.80,  the 
other,  $3.90,  in  gold  per  ton.  Upon  inspecting  the  sam- 
ples from  other  cuts,  carefully,  I  found  the  gold  occurred 
in  small  wires,  and  that  these  wires  were  sharp  and  very 
easily  penetrated  the  cotton  duck,  so  that  a  form  of  con- 
centration was  being  effected  the  more  the  sample  was 
rolled.  To  obviate  the  trouble  the  large  samples  were 
placed  in  a  gunny-sack,  brought  to  the  surface  and  dried, 
and  a  canvas  having  a  prepared  or  soft  enameled  sur- 
face was  used  thereafter  in  all  work  involving  gold  and 
silver  determinations.  The  enamel  of  the  cloth  admits  of 
a  thorough  admixture  of  the  sample,  and  at  the  same  time 
prevents  the  gold  from  adhering  to  the  canvas — the  ac- 
tion being  similar  to  that  of  glazed  paper  in  the  laboratory. 

I  think  it  a  great  mistake  not  to  have  some  means  of 
identifying  a  sample  after  it  has  been  closed  and  sealed, 
and  a  serial  number,  not  necessarily  the  sample  number, 
should  always  be  placed  on  the  bag  as  soon  as  the  sample 
is  sacked  and  tied. 

Richard  A.  Parker. 

Boston,  June  20,  1903. 


The  Editor: 

Sir. — Mr.  Richard  A.  Parker,  in  your  issue  of  July  4, 
touches  upon  a  subject  of  interest  to  those  whose  duty  it 
becomes  sometimes  to  sample  auriferous  deposits. 

Mr.  Parker's  meaning,  in  the  letter  referred  to,  is  not 
altogether  clear.  Does  he  mean  that  the  concentration 
of  the  gold  in  the  interstices  of  the  duck  took  place  during 

—151— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

the  rolling  of  the  sample,  which  had  been  broken  up  to 
pieces  about  ^-in.  size,  or  after  the  final  quarterings  had 
been  crushed  to  about  >^-in.  ? 

If  the  former  was  the  case,  how  was  the  gold  trans- 
ferred from  y2-\n.  pieces  of  a  wet  and  sticky  quartzose 
schist  into  the  pores  of  an  ordinary  sampling  cloth  ? 

If  the  concentration  took  place  after  the  final  quarter- 
ings, how  does  it  come  that  those  quarterings  assayed 
$14.80  and  $3.50,  respectively?  Why  was  not  the  concen- 
tration equal  in  each  of  the  quarterings  ? 

The  question  of  gold  being  removed  from  samples  and 
caught  in  the  interstices  of  a  sampling  canvas,  is  an  old 
one.  The  writer  had  an  experience  of  a  similar  nature 
some  years  ago.  The  promoter  of  a  gold  mining  prop- 
erty urged,  as  the  cause  of  extremely  low  results  obtained 
from  a  sampling  of  the  various  openings,  that  the  writer 
had  broken  down  the  samples  onto  canvas,  and  quartered 
them  down  on  same,  rolling  them  over  and  over  as  many 
as  twenty  times  or  more.  He  enclosed  with  his  letter  a 
piece  of  the  canvas  to  show  how  easily  the  gold  could  be 
caught  in  it.  He  failed,  however,  to  explain  how  the  gold 
got  out  of  the  quartz,  for  the  samples  were  large  ones,  and 
the  material  was  broken  to  about  the  size  of  hickory  nuts. 

W.  L.  Austin. 

New  York,  July  6,  1903. 


The  Editor: 

Sir. — In  your  issue  of  July  4,  1903,  Mr.  Parker  gives 
an  example  of  the  very  indifferent  results  obtained  by  roll- 
ing and  quartering  samples  on  canvas  sheets. 

Every  one,  of  course,  admits  that  quartering  samples  on 
a  canvas  is  not  the  most  reliable  way  of  obtaining  per- 
fectly accurate  results.  Still,  away  from  railroads  where 
one  is  obliged  to  pack  for  days,  riffles  not  being  made,  as  a 
rule,  substantial  enough  to  stand  rough  handling,  nothing 
is  left  to  the  engineer  but  the  use  of  a  canvas  sheet  which 
can  be  replaced  anywhere  when  necessary. 

—152— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

In  the  case  cited  by  Mr.  Parker,  and  where  he  found 
such  difference  between  his  two  ultimate  quarterings  as  to 
render  the  results  useless,  I  hardly  think  that  it  was  due 
to  concentration  on  the  canvas,  but  that  the  cause  lay  with 
the  nature  of  the  sample  itself.  Mr.  Parker  states  that 
the  ore  was  a  quartzose  schist,  "slightly  wet  and  a  little 
sticky."  Naturally,  if  the  sample  carried  fine  metallics 
which  were  set  free  by  the  partial  crushing, these  particles, 
as  a  matter  of  course,  would  be  picked  up  by  the  larger 
"wet  and  sticky  pieces"  of  the  samples,  which  pieces  acted, 
in  fact,  as  clay  does  in  a  sluice.  Under  such  conditions  a 
fair  sample  can  hardly  be  expected.  As  stated  above,  we 
all  recognize  that  canvas  quartering  is  far  from  being 
accurate,  but  still  very  close  checking  can  be  done  with  it. 

The  example  given  further  on,  although  not  at  first  in- 
tended to  check  canvas  results,  will,  however,  give  an  idea 
of  how  close  results  can  be  obtained  by  using  ordinary 
precautions. 

The  following  samples  were  taken  from  a  quartz  vein, 
ribboned  in  structure ;  the  quartz  was  middling  hard  and 
carried  small  amounts  of  sulphides  of  iron,  zinc,  lead  and 
copper.  Three  mill-runs  of  twelve  hours  each  gave  re- 
spectively 80,  83.34  and  87  per  cent  of  the  gold  values 
on  the  plates ;  consequently  the  ore  could  almost  be  called 
free  milling,  notwithstanding  the  small  amount  of  sul- 
phides present.  The  samples  weighed  originally  30  to  60 
or  70  lb.,  according  to  width  of  vein,  and  after  being  thor- 
oughly dried  they  were  crushed  by  hand  to  about  quarter- 
inch  size,  and  then  thoroughly  rolled  and  quartered  down 
on  a  large  piece  of  canvas,  the  owner  of  the  mine  securing 
the  two  rejected  quarters  of  the  sample. 

The  samples  were  assayed  in  Denver  by  a  well  known 
and  reliable  assayer  and  gave  the  results  below,  or  an 
average  of  gold  2.333  foot-ounces,  and  silver  8.45  foot- 
ounces.  Later  I  received  the  results  from  the  mine  own- 
er, as  giving  an  average  of  gold  2.42  foot-ounces,  and 
silver  10.4  foot-ounces. 

—153— 


SAMPLING    AND    ESTIMATION    OF   ORE   m    A    MINE 

Mine  Owner's  Engineer's 

Samples.  Samples. 

No.  Ag.  oz.  Au.  oz.  Ag.  oz.      Au.  oz. 

1 5-6o  0.87               4.13  0.87 

2 13.70  1. 16  12.00  I.OO 

3 326  0.44                1.33  0.40 

4 17.38  1.31  16.00  1.28 

5 6.12  4.52                3.72  5.08 

6 3.80  1.74               2.00  1.40 

7 5.48  0.64               4.00  0.56 

8 13.12  7.58  12.40  8.00 

9 16.42  I.07  15.00  0.88 

10 6.61  1.04               2.47  0.93 

II.. 8.05  5.78               3.42  m8 

12 4.17  0.35               3.00  0.39 

13 7.52  1.32               420  1.20 

14 15.60  1.45  17.00  1.26 

The  difference  in  the  gold  average  of  the  two  sets  of 
samples  is  4  per  cent,  which  is  close  enough  for  all  prac- 
tical purposes;  but  the  difference  in  silver  of  19  per  cent 
is  rather  large.  However,  taking  into  consideration  that 
the  assays  from  the  mine  owner  were  made  away  from  any 
railroad,  in  Mexico,  where  supplies  are  not  always  stand- 
ard, and  also  taking  into  consideration  that  his  interest 
in  the  matter  might  have  made  him  perhaps  rather  partial 
to  the  mine,  the  above  results  show  that  pretty  accurate 
work  can  be  done  on  the  canvas  sheet,  when  one  is  obliged 
by  circumstances  to  use  it.  Ordinary  care  should  be 
taken,  samples  should  naturally  be  well  dried  and  broken 
to  about  34 -inch  size,  then  carefully  rolled  so  as  to  obtain 
a  perfect  mixture ;  if  the  mass  slides  instead  of  rolling  on 
the  canvas  the  ultimate  quarters  will  be  spotted,  as  a  con- 
centration in  that  case  occurs;  after  each  quartering  the 
sheet  should  be  thoroughly  brushed  with  a  stiff  broom  and 
close  checking  of  the  quarter's  assays  will  invariably  fol- 
low. 


G.   M.   GOUYARD. 


Denver,  Colo.,  July  t.^,  1903. 


The  Editor : 

Sir. — In  answer  to  Mr.  W.  L.  Austin's  queries  in  the 
Journal  of  July  11,  I  would  say  that  the  original  sample 

—154— 


SAMPLING    AND    ESTIMATION    OF    OKK    IX    A    MINK 

was  broken  to  about  Yi  in.  before  quartering  and  rolling 
began  ;  the  final  samples  were  broken  to  between  J4  and  ^ 
in.  In  a  more  or  less  clayey  mass  which  had  been  rolled 
frequently  it  is  impossible  to  say  when  the  concentration 
took  place,  if  indeed  it  was  a  concentration ;  the  more  the 
sample  w^as  rolled  wet  as  taken  from  the  face,  the  greater 
the  tendency  of  the  gold  wires  to  find  lodgment  in  the  soft 
duck,  or  in  the  clayey  film  that  readily  forms  after  fre- 
quent rollings  of  such  samples. 

The  differences  in  assay  value  were  due  to  the  greater 
number  of  gold  particles  in  one  sample  than  the  other,  and 
the  reason  these  particles  were  not  more  uniformly  dis- 
tributed by  rolling  was  due  to  the  sticky  nature  of  the  ma- 
terial sampled. 

Any  one  at  all  familiar  with  the  auriferous  schists  of 
the  Mother  Lode  of  California  would  readily  see  the  im- 
portance of  the  suggestion  made:  the  difference  between 
ordinary  gold  quartz  and  the  schists  named  is  apparent 
and  readily  accounts  for  the  greater  ease  with  which  the 
gold  particles  leave  the  sample  in  the  latter  case. 

The  point  I  wish  developed  is  the  danger  of  quartering 
down  ores  carrying  free  gold,  wet  or  damp,  and  doing 
this  upon  the  ordinary  canvas  such  as  is  generally  used 
for  such  purpose ;  it  is  difficult  to  properly  clean  the  canvas 
after  each  sampling  owing  to  the  sticky,  clayey  material 
that  fills  the  interstices  of  the  duck,  and  particles  of  gold 
left  from  one  sample  may  be  incorporated  in  the  following 
one,  thus  rendering  one  too  high  and  the  other  too  low  by 
the  value  of  the  gold  misplaced.  The  samples  should  be 
fairly  dried  and  then  quartered  upon  canvas  having  an 
enameled  surface. 

RicHAM)  A.  Parker. 

Saranac  Lake,  N.  Y.,  July  17,  1903. 


The  Editor: 

Sir. — In  experimental  physics  it  is  often  necessary  to 
know  how  far  the  results  are  accurate  after  all  positive 
corrections  have  been  applied.       The  method   of  least 


—15s- 


SAMPLING    AND    ESTIMATION    OF    ORE>  IN    A    MINE 

squares  is  a  mathematical  process  which  accompHshes  this 
by  combining  the  Hmiting  inaccuracies  of  the  instruments 
and  methods  employed,  and  gives  a  figure  representing 
the  mean  probable  error  of  the  final  result. 

In  a  mine  only  a  very  small  portion  of  the  ore  is  ex- 
posed to  the  sampler,  and  as  the  values  are  very  unevenly 
distributed  the  mean  probable  errors  are  very  large,  as 
compared  with  those  of  other  methods  of  measurement 
which  we  have  to  make.  In  a  great  many  cases,  the  cost 
of  mining  and  treatment  being  deducted,  there  is  only  a 
very  small  margin  of  profit  left,  and  it  is  then  important 
that  the  engineer  should  be  able  to  represent  to  the  owners 
or  prospective  purchasers  how  far  the  mine  is  a  safe  in- 
vestment and  how  far  a  speculation. 

If  a  block  of  ore  is  sampled  and  said  to  average  A, 
we  mean  this  as  the  nearest  approach  to  its  true  average, 
which  can  be  obtained  by  taking  samples  around  its 
boundaries ;  the  figure  A  may  be  somewhat  too  great  or  it 
may  be  somewhat  too  small.  To  be  precise,  therefore,  we 
should  say  the  average  is  A  =fc  V,  where  V  is  the  mean 
probable  error.  Now  suppose  we  have  several  blocks 
constituting  a  mine  and  averaging  A^  ±  V^,  A^  db  Vg,  A3 
±  V3,  etc.,  and  weighing  respectively  Mj,  Mo,  M3,  etc.; 
then  by  the  method  of  mean  squares  the  average  of  the 
whole  mine  will  be 

M,  A,  4-  M,  A,  +  M,  A,  -f-  etc. 
Ml  +  M2  +  M3  +  etc. 

,    \MSVS  4-  M^V^,  +  M^sV\ 
—  Ml  +  M2  +  M3  +  etc.  ' 

which  may  be  more  conveniently  written 


S   M  A    4.    \    ^  M^  V^ 
S   M      —  S  M 

The  following  is  a  tabulated  example: 

M  A+ V 

Tons.  Ozs.  Per  Ton.  M  A.  M*V«. 

Block    No.    I 2,100         3.60  +.32  7.560  451.594 

Block    No.    2....  5,800         4-50   +.20  26,100  1,345,600 

Block    No.    3....    1,200         6.20  -f- .55  7,440  435,600 


Totals    9.100  41,100         2.232,794 


SAMPLING   AND    ESTIMATION    OF    ORE    IN    A    MINE 


Mean 


41,100 
"9T100" 


+ 


or  4  51  +  0.16  oz.  per  ton. 

The  mean  probable  error  involved  in  the  usual  method 
of  sampling  cannot  be  deduced  in  any  very  rigorous 
mathematical  manner,  but  requires  certain  broad  assump- 
tions.    Suppose  a  square  to  be  divided  into  eight  equal 


triangular  portions  so  that  each  presents  an  equal  and 
similarly  situated  sampling  face  as  shown  in  Fig.  i,  and 
that  two  samples  assaying  a  and  b,  are  taken  on  each 
such  face;  then  we  may  assume  the  average  for  the  tri- 
angle, as  far  as  can  be  ascertained  from  the  face,  to  be 

niaa  4"  nihb       ,      iriaa  —    nibb 
ma  +  mb         —        ma  +  nib 

for  each  triangle,  where  nia  and  mb  are  respectively  the 
thicknesses  of  the  vein  at  a  and  b.    The  term 

_L     rriaa  —   mhb 
-^       ma  -h  mb 

represents  the  variation  from  the  simple  average  to  just 
the  same  accuracy  as 

maa  4"  mbb 
ma  +  mb 

represents  that  average. 

If  we  now  compound  the  averages    of  the  triangle  by 
the  method  of  least  squares,  we  get  for  the  whole  block : 

—157— 


SAMPLING    AND    ESTIMATION    OF    ORK    IN    A    MINE 


,  Z(maa  +  mbb)      ,    \   S(inaa  — mbb)' 

A±V  -      s(ma+mb)     —  S(ma+mb) 

When  a  block  of  ore  is  not  square  it  may  be  divided 
in  a  similar  manner,  as  shown  in  Fig.  2.  Greater  accu- 
racy (or  a  smaller  variation  from  the  average)  is  obtained 
by  dividing  off  the  ore-shoots  separately  where  their 
boundaries  are  reasonably  well  defined ;  the  results  can 
afterward  be  compounded  with  those  of  other  blocks  of 
pay  ore. 

If  the  one  below  the  bottom  level  is  divided  off  into 
two  45**  right-angled  triangles,  as  in  Fig.  2,  a  large  value 
is  obtained  for  V.  This  is  illustrated  by  the  following 
table,  which  also  shows  the  method  of  deducing  the  mean 
variation  where  more  than  two  samples  to  the  triangle 
are  taken. 

Sample 
No. 


Variation  from 

ATidth 

Assay 

Oi.-Ft. 

simple  mean 

Ft. 

oz.  per  ton 

per  ton 

of  ma 

(m) 

(a) 

(ma) 

(5.730— 1.787&C.) 

2.6 

2.20 

5.720 

3.933 

1.9 

2.05 

3.895 

2.108 

0.6 

1.09 

.654 

I.I33 

1.3 

•05 

.065 

1.722 

1.2 

.07 

.084 

1.703 

1.7 

.18 

.306 

1. 381 

I  Totals 10.724  11.980 

I  Means 1.787  1.997 


uS5 


7 2.0  1.22  2.440  0.279 

8 2.6  i.oo  2.600  O1439 

9 2.3  0.30  .690  1471 

10 1.6  0.80  1.280  0.881 

II 0.9  0.41  .369  1,792 

12 I.I  5-o8  5.588  3.427 


2  Totals r 19.8  12.967  8.289 

2  Means 1.65  2. 161  1.381 

From  which  we  deduce 


[0.724  4-  12.967 

A     =      - 

and 


A    =        '  "   ^^.8  =     1-197 


V    =    :fcVii-98o'+  8.289'     ^    +  0.736 
19.8  ~ 


—158— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 


.«52 


The  law  of  least  squares  only  applies  strictly  where  V 
is  small  compared  with  A,  but  in  extreme  cases,  such  as 
the  above,  V  is  none  the  less  useful  as  indicating  the  low 
degree  of  accuracy  of  A ;  and  although  a  more  complex 
law  may  be  used  for  these  cases,  it  is  obvious  that  V  can- 
not be  expected  to  be  more  accurately  ascertained — with 
the  same  data — than  A. 

No  attempt  is  here  made  to  estimate  the  probable  quan- 
tities of  ore  beneath  the  bottom  level ;  we  are  dealing  only 
with  the  degree  of  variation  of  value  of  given  quantities 
of  ore- 


-IS9— 


SAMPLING    AND    ESTIMATION    OF    ORE    ^N    A    MINE 

If  greater  certainty  is  required  on  a  smaller  amount  of 
ore  below  the  bottom  level  the  assays  may  be  divided 
among  several  smaller  triangles,  which  will  be  equivalent 
to  a  row  of  rectangles  of  the  same  areas  and  bases.  A 
will  then  remain  the  same,  but  V  will  be  considerably  re- 
duced, and  this  should  be  done  if  the  averages  are  to  be 
combined  with  those  of  ore  better  blocked  out.  The  same 
remarks  may  also  apply  to  ore  exposed  on  two  sides ; 
rectangles  may  be  used  instead  of  rectangular  triangles, 
except  at  corners,  but  they  should  not  vary  much  in 
dimensions  from  the  ratio  of  2  to  i,  except  in  the  cases 
where  ore  or  barren  ground  evidently  runs  in  shoots  or 
horizontal  zones. 

The  averages  of  all  the  ore  considered  payable  having 
been  combined  in  classes  according  to  average  value,  mean 
error  and  other  considerations  and  the  method  of  min- 
ing and  treatment  having  been  decided  upon,  the  value 
of  the  mine  may  be  investigated  by  examining  them  sev- 
erally by  the  following  formulae: 

I    =     ^(A±V)_^ 

Where  $  is  the  value  in  money  of  an  ounce  or  unit  of 
the  crude. 

p  is  the  percentage  extraction. 

q  is  the  tonnage. 

M  is  the  sum  of  costs  of  mining,  machinery,  treatment 
and  bank  rate  of  interest  compounded  on  the  time  the  ore 
is  to  stay  in  the  ground. 

As  regards  presentation  to  the  financier.  I  find  the 
word  "risk"  is  fairly  well  understood  on  Wall  Street :  5 
per  cent  risk  is  an  even  chance  of  making  or  losing  5  per 
cent  on  the  investment  and  100  per  cent  risk  means  an 
even  chance  of  doubling  or  losing  the  investment.  There- 
fore, if  R  represents  the  percentage  risk,  it  is  given  by 

^    "  I 

which  becomes  an  appj''^ciable  figure  even  in  the  best  de- 
veloped mines. 

— 160 — 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

Purely  business  men  in  the  ordinary  competition  of 
trade  do  not  expect  to  make  more  than  bank  interest  on 
their  capital  and  their  salary  as  clerks  without  running 
some  risk,  but  they  make  money  in  the  long  run  by  sound 
judgment  of  the  comparative  chances  of  profit  and  loss 
and  by  seizing  the  opportunities  for  the  greatest  probable 
profit  at  the  least  risk.  The  engineer  who  does  not  take 
proper  account  of  the  high  assays  obtained  in  fair  sam- 
pling is  therefore  a  poor  adviser  to  the  financier. 

The  use  of  the  compound  system  of  averages  does  away 
with  the  necessity  for  the  use  of  such  indefinite  and  arti- 
ficial terms  as  ''ore  in  sight,"  "probable  ore,"  etc. ;  not  only 
do  the  number  of  sides  exposed  count  in  determining  the 
average,  but  the  spotty  character  of  the  ore,  the  size  of  the 
ore  blocks  and  the  extent  of  the  mine  development  are 
all  properly  compounded  in  the  value  V  and  make  them- 
selves known  to  the  capitalist  as  the  value  R  which  he 
fully  appreciates,  and  which  ought  also  to  be  a  guide  to 
the  engineer  in  recommending  that  the  installment  of  ma- 
chinery be  immediate  or  deferred  until  the  mine  is  further 
developed. 

Just  as  much  reliance  can  be  placed  upon  V  as  a  meas- 
ure of  the  probability  (or  degree  of  **insightedness")  of 
the  ore  as  upon  A,  for  whereas  A  has  its  origin  in  values 
of  single  samples  which  are  very  variable,  so  V  is  built 
up  of  differences  of  values  of  single  samples  which  are 
not  much  more  variable  quantities. 

I  have  used  these  rules,  with  modifications  to  suit  local 
conditions,  for  over  tw'o  years  upon  every  set  of  samples 
I  have  come  in  contact  with,  and  now  place  absolute  con- 
fidence in  the  reliability  of  the  results. 

Blamey  Stevens. 

Valdez,  Alaska,  July  21,  1903. 


The  Editor: 

Sir. — In  the  perusal  of  the  articles  on  the  estimation 
and  sampling  of  ore-bodies  which  have  appeared  in  your 

—161— 


SAMPLING   AND    ESTIMATION    OF    ORE   IN.^  A    MINE 

paper  recently,  and  which,  by  the  way,  have  afforded  me 
much  interest  and  pleasure,  I  have  not  noticed  any  men- 
tion made  of  the  prismoidal  formula  in  its  application  to 
the  measurement  of  irregular  ore-bodies.  Some  years 
since  I  discussed  the  value  of  it  in  one  of  a  series  of  papers 
on  mine  surveying  which  I  wrote  for  Mines  and  Minerals. 
The  application  of  it  is  so  simple  and  its  results  are  so 
accurate  in  practice  that  I  should  think  its  usefulness 
would  appeal  to  mining  engineers,  as  it  always  has  to  civil 
engineers. 


Naturally  care  must  be  exercised  in  its  use ;  to  illustrate, 
take  a  body  acgefhdb,  as  shown  by  the  accompany- 
ing diagram,  the  calculation  would  need  to  be  made  first 
for  the  smaller  volume,  a  c  d  b,  then  for  c  g  e  f  h  d. 

Re-entering  angles  must,  of  course,  also  be  taken  into 
account;  but  the  three  sections  of  a  body  being  given — 
top,  middle  and  bottom — their  areas  can  be  quickly  ascer- 
tained by  dividing  the  sections  into  triangles.     The  vol- 

—162— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

ume  of  the  body,  calling  the  top  section  a,  middle  section 
a',  bottom  section  a"  and  height  h,  is : 
a  +  4a'  +  a" 
6 ^ 

I  know  by  frequent  use  both  in  the  estimation  of  ore  in 
mines  and  in  piles,  that  it  gives  results  accurate  enough 
for  practical  purposes. 

Another  point  I  would  like  to  bring  up  is  the  protec- 
tion of  mine  samples.  The  value  of  accurate  measure- 
ment and  sampling  of  ore-bodies  is  self-evident,  but  to  my 
mind  the  protection  of  the  sample  from  mine  to  assay 
office  is  at  least  of  equal  importance.  A  device  which  has 
given  great  satisfaction  to  me,  and  which  I  have  used  for 
years,  is  a  lead  seal  like  that  used  by  the  express  com- 
panies. The  seal  is  of  lead  ^  in.  diameter,  34  ii^-  thick, 
with  two  holes  through  its  circumference  large  enough 
for  a  thin  copper  wire  to  pass  through.  When  the  sample 
bag  has  been  tied  with  the  wire  passed  through  the  seal 
the  holes  in  the  latter  are  closed  by  pressure  of  a  die  with 
a  name  sunk  on  one  side  and  some  mark  or  monogram  on 
the  other.  Any  novelty  company  will  make  such  a  die, 
and  usually  has  the  lead  seals  for  sale.  Samples,  thus 
sealed,  and  locked  in  sole  leather  bags,  also  sealed,  are 
safe. 

The  advantage  of  using  this  method  as  compared  to 
sealing-wax  is  also  apparent,  where  sampling  is  done  in 
open-cuts  or  draughts. 

AUGUSTE  MaTHEZ. 

New  York,  August  12,  1903. 


The  Editor: 

Sir. — I  have  followed  with  great  interest  your  series  of 
articles  on  "The  Sampling  and  Estimation  of  Ore  in  a 
Mine,"  and  with  your  permission  will  make  some  com- 
ment thereupon  from  the  standpoint  of  the  practice  fol- 
lowed in  the  group  of  mines  of  which  I  am  the  technical 
head. 

—163— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN^    A    MINE 

The  function  of  sampling  is  now  well  recognized  on 
these  gold-fields  as  one  of  the  most  important  contribu- 
tions to  the  commercial  success  of  the  mines.  The  old- 
time  haphazard  method  of  promiscuous  sampling,  either 
by  taking  pieces  of  ore  from  trucks  or  "picking"  down  odd 
sections  in  the  mine,  has  no  present  adherents,  nor  is  the 
work  of  sampling  relegated  to  the  inexperienced  "new 
chum,""^-  who  formerly,  if  not  thought  intelligent  enough 
for  any  other  class  of  work,  was  provided  with  a  sampling 
job.  The  enormousl}-  valuable  part  played  in  the  economy 
of  mining  by  systematic  and  conscientious  sampling  is 
now  well  recognized.  A  class  of  skilled,  intelligent,  alert 
men,  culled  largely  from  trained  engineers,  has  arisen,  and 
these  have  raised  the  status  of  mine  sampler  from  infe- 
riority to  one  of  importance.  It  is  gratifying  to  read  that 
the  peculiar  and  constant  calls  upon  the  integrity  and 
will  of  the  sampler  have  been  duly  acknowledged  in  the 
articles  under  review,  and  I  believe  the  mine  sampler  will 
be  encouraged  and  assisted  to  still  greater  efforts  because 
of  them. 

In  your  introductory  article,  you  dwell  upon  the  fact 
that  methods  of  mine  valuation  and  examination  have 
undergone  very  considerable  development  during  the  past 
decade,  and  you  indicate  the  possibility  that  the  close 
attention  to  this  part  of  gold  mining  business  on  the  Rand 
has,  to  some  extent,  influenced  for  the  better  the  mining 
industry  at  large.  Speaking  as  an  Australian,  and  being 
also  personally  acquainted  with  American  and  European 
methods  of  some  ten  years  since,  I  can  cordially  indorse 
the  views  which  you  have  expressed  as  to  the  present  ad- 
vanced stage  reached  in  the  practice  of  mine  sampling  and 
valuation  compared  with  the  period  I  have  named.  Nor 
does  the  advance  relate  only  to  other  countries ;  it  is  true 
no  less  of  the  Rand.  The  systems  now  in  vogue  repre- 
sent the  outcome  of  evolutionary  growth,  and  are  based 
upon  experience  and  sound  principles ;  but  the  writer  well 
remembers  some  few  years  ago  having  to  deal  with  assay 
reports  and  plans  which  were  prepared  and  averaged  in 

*This   is  the   colonial   equivalent   of   our   Western   "tenderfoot." — Editor. 
— 164 — 


SAMPLING    AND    ESTIMATION    OF    ORE   IN    A    MINE 

the  most  elementary  fashion,  without  any  regard  to  widtlis 
of  reef  or  intervals  between  sections.  Now,  happily, 
these  instances  are  rare,  and,  in  fact,  so  far  as  the  Rand  is 
concerned,  things  of  the  past ;  and  the  engineer  is  no 
longer  troubled  by  enormous  discrepancies  between  assay 
reports  and  milling  returns,  making,  of  course,  the  allow- 
ances which  experience  dictates  as  necessary  between  mere 
assays  and  actual  recovery. 

The  engineer  practicing  on  the  Rand  recognizes  that 
here  is,  par  excellence,  the  field  which  lends  itself  to  the 
application  of  method.  The  banket  beds,  while  fluctuating 
violently  both  in  width  and  value  in  local  areas,  are  unpar- 
alleled for  consistency  when  considered  throughout  the 
Witwatersrand  area,  and  the  great  range  of  development 
made  possible  by  the  natural  condition  of  the  ore  and  gold 
deposition  give  quite  unusual  facilities  for  estimation  and 
the  application  of  the  law  of  averages.  That  these  facili- 
ties have  been  recognized  and  embraced  is  undoubtedly 
creditable  to  the  engineers,  past  and  present,  on  the  Rand, 
but  it  cannot  be  gainsaid  that  the  conditions  were  such  as 
to  make  the  scheme  almost  self-suggestive.  In  looking 
back  over  one's  past  experience  in  other  countries,  and 
taking  the  natural  conditions  generally  present  in  quartz 
deposits  into  account,  whether  these  have  been  exfiltrated 
into  previously  existing  fissures  or  segregations  within 
rock  masses,  and  considering  further  the  small  amount  of 
development  done  upon  the  "prospect"  which  one  was 
engaged  to  examine  and  value,  and  at  the  same  moment 
transporting  oneself  mentally  to  the  mines  of  the  Rand, 
the  conclusion  is  forcibly  impressed  upon  one's  mind  how 
exceedingly  advantageously  situated  is  the  man  working 
on  these  fields,  and  how,  in  comparison,  the  risks  of  error 
he  runs,  given  careful  work,  are  minimized  to  the  last 
degree  in  comparison  with  the  engineer  who  is  employed 
on  deposits  of  a  different  character. 

Mine  examination  in  South  Africa  is  shorn  generally 
of  many  of  the  dangers  present  in  pursuing  similar  inves- 
tigations either  in  America  or  Australia.  The  mine  Salter, 
with  his  many   ingenious  methods   of  introducing  gold 

— I6s— 


SAMPLING   AND    ESTIMATION    OF    ORE    fN    A    MINE 

either  in  the  ore  face  or  into  samples,  is  unknown.  The 
assayer  who  returns  high-grade  results  from  ''grindstone" 
specimens  has  not  flourished.  The  reason,  probably,  why 
there  is  no  room  for  people  of  this  class  is  that  banket 
mining  is  essentially  a  capitalist's  operation. 

In  quartz,  or  in  other  forms  of  ore-deposit,  the  metal 
worked  may  occur  in  rich  bunches,  which  enable  a  pros- 
pector of  limited  means  to  collect  sufficient  of  it  to  pay 
his  way.  In  banket  this  is  not  the  case,  and  therefore 
the  banket  bearing  ground  is  sold  on  option  to  a  company 
financially  strong  enough  to  exploit  it.  In  the  case  of  the 
examination  of  mines,  the  personal  incentive  to  defraud 
for  profit  is  wanting,  because  the  mine  is  owned  generally 
by  a  company,  and  individuals  are  not  interested  suffi- 
ciently to  make  it  worth  their  while  to  seek  means  to  create 
results  more  favorable  than  are  actual.  No  claim  can 
therefore  be  made  that  persons  connected  with  the  gold 
mining  business  here  are  more  scrupulous  than  elsewhere. 
The  simple  fact  is  that  opportunity  seldom  arises  for  an 
individual  to  perpetrate  fraud  for  his  personal  advantage, 
owing  to  the  special  conditions  of  the  case,  and  hence, 
having  no  temptation  to  err,  all  are  alike  honest. 

I  cannot  quite  agree  with  you,  ^Ir.  Editor,  in  your  state- 
ment that  "average  value  of  the  ore  in  the  past  can  be 
ascertained  from  the  records  of  a  mine,"  if  by  "ore"  you 
intend  to  convey  the  larger  meaning  of  the  exhausted  por- 
tion of  the  mine.  In  my  experience,  I  have  found  it  just 
as  necessary  to  make  an  inspection  of  the  exhausted  por- 
tion of  the  mine  as  of  the  current  producing  section,  for 
the  following  reasons :  Firstly,  to  prove  to  my  own  satis- 
faction that  the  mine  has  been  fairly  worked  in  regard  to 
area  of  reef.  It  will  readily  occur  to  the  reader  that  a 
mine  can  be  easily  manipulated  to  show  high  returns  over 
a  short  period.  Take  the  case  of  a  mine  which  has  two 
widely  separated,  narrow,  rich  shoots  of  ore.  On  such  a 
proposition  it  would  be  possible  to  develop  for  a  period  of 
one  year,  and  at  the  expiration  of  that  time  commence 
milling  on  high-grade  rock.  Assume  that  during  the 
twelve  months  100,000  tons  of  ore  have  been  developed 

—166— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

at  a  cost  of  £25,000,  but  that  actually  the  rich  pay  ore 
represents  only  25  per  cent  of  the  total  tonnage.  Then 
the  charge  for  the  redemption  of  the  ore  would  have  to 
appear  in  the  working  account  as  at  4  by  5,  or  20s.  per 
ton  milled,  and  it  is  obvious  that  such  a  charge  would 
require  unusually  rich  rock  to  bear  it.  It  would  also  be 
doubtful  if  a  mine  yielding  so  low  a  percentage  of  pay- 
rock  could  be  developed  fast  enough  to  supply  any  but  a 
mill  of  very  small  capacity. 

The  second  reason  why  it  is  important  to  examine  for- 
mer workings  in  a  mine  under  examination  is  to  find  out 
the  relation  between  reef  width  and  stope  width.  It  has 
occurred  in  the  writer's  experience  that  the  grade  of  a 
mine  showed  much  lower  than  its  mean  average,  because 
of  including  unpayable  bands  of  reef  or  waste  in  the  stopes 
for  the  purpose  of  showing  low  working  costs. 

Now,  it  is  indisputable  that  mines  should  be  operated 
for  profit  per  ton,  and  such  an  objective  as  low  operating 
costs  is  quite  unjustifiable,  excepting  after  the  considera- 
tion of  the  main  issue  stated.  I  believe  it  will  be  agreed 
therefore  that  **the  average  value  of  the  ore  in  the  past" 
is  not  a  criterion  for  the  engineer  to  accept  unquestion- 
ingly ;  otherwise,  in  the  first  instance  quoted,  he  would  be 
liable  to  have  his  judgment  of  the  existing  development 
prejudiced  too  favorably,  and  in  the  second  instance  the 
past  records  might  lead  him,  even  in  the  light  of  present 
satisfactory  development,  to  put  much  too  low  a  value  on 
the  mine. 

Mines  in  New  Districts. — You  have  plainly  indicated 
some  of  the  precautions  to  be  observed  by  the  engineer  in 
estimating  values  of  undeveloped  mines  in  new  districts, 
all  of  which  are  pertinent.  It  seems  to  me  that  one  chief 
difficulty  in  estimating  values  in  mines  so  placed  is  the 
want  of  facilities  for  observation.  The  engineer  is  called 
upon  to  decide  whether  a  certain  "prospect"  is  valuable  or 
otherwise,  and  the  available  data  upon  which  his  judg- 
ment and  previous  experience  are  brought  to  bear  are  often 
meager  in  the  extreme.  Perhaps  the  points  exposed  are 
widely  separated ;  he  has  no  opportunity  of  studying  dike 

—167— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 


Form  No.  99. 


s5 


Meyer  &  Charlton  Gold 


n^HSTE       V 


Shafts ^J%^4A.  ..^Reef 


DtsTANCi  rROH. 


Assay. 

of 

Sections 


dwtS. 


S  »3  a. 
dwts. 


Assay. 
Inches. 


/At  9 
/dV 


/^ 


V 


^o 


Jc 


4^£> 


^^ 


/6' 

^/ 

J 

/3 


/a 
// 

/^ 
/^ 


r 

9- 

/• 

J' 
J'6- 
/   S 
// 


^6 


PJ2 


P6 


/^ 


d"? 


3fU 


C-  ff- 


^7 


/•sT 


7 


7-2- 

PS- 

-?^- 
4,5- 


—168— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

Mining  Company,  Limited. 


^XjTJES 


^^^ L  evel-- M^<?-<r^ Drive 

Date-^ -^-C  ///4^ 


— 169 — 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

or  fault  effects,  nor  the  probable  width  or  trend  of  the 
shoot;  nor,  in  fact,  any  of  the  vital  factors  entering  into 
the  ultimate  consideration  of  the  ^/alue  of  the  mine  as  a 
commercial  undertaking.  He  is  therefore  forced  to  one  of 
two  alternatives,  either  (i)  to  intuitively  sum  up  the 
'^alue  of  the  property,  or  (2)  to  refuse  to  commit  himself 
to  an  opinion  until  more  work  is  done.  The  first  alterna- 
tive will  always  be  avoided  by  the  judicious  engineer  in 
favor  of  the  second,  both  in  his  own  interest  and  in  that 
of  his  client ;  nor  is  it  consistent  with  sound  professional 
practice.  The  new  district  is  always  fertile  in  the  class 
of  property  that  is  alleged  to  yield  ore  of  the  same  grade 
as  that  in  some  adjacent  successful  district.  In  this  coun- 
try, for  instance,  it  is  usual  to  find  the  prospector  or  claim- 
holder  filled  with  the  conviction  that  as  the  working  costs 
on  the  Witwatersrand  approximate  to  6  dwts.  of  fine  gold 
per  ton  milled,  he  has  only  to  establish  the  fact  that  he 
has  7  dwts.  assay  value  in  his  ore  to  claim  payability.  He 
conveniently  forgets,  or  probably  does  not  know  that 
from  an  assay  value  of  7  dwts.  he  can  only  recover  by  the 
ordinary  processes  4.2  dwts.,  which  would  leave  him  1.8 
dwts.  on  the  wTong  side,  figuring  costs  at  6  dwts.  per  ton. 
The  engineer  has  therefore  to  be  careful  when  fixing  the 
value  of  a  proposition  to  allow,  as  a  margin  of  safety,  that 
only  60  per  cent  of  the  original  assay  will  be  recovered. 
And  here  I  would  plead  for  the  general  expression  of 
values  in  terms  of  currency,  as — in  speaking  of  gold — 
ounces  may  be  fine  ounces,  worth  84s.  iid.,  or  bullion, 
worth  50s.  per  ounce ;  and  in  all  cases  the  estimated  recov- 
ery values  of  ore  per  ton  treated  should  be  given  and 
expressed  as  so  much  money  value  per  ton. 

The  Cost  of  Mine  Examination. — I  observe  w^ith  satis- 
faction that  you  have  drawn  attention  to  the  cost  involved 
in  mine  examination.  In  two  recent  instances  of  developed 
mines  sampled  under  the  writer's  supervision  the  number 
of  sections  totaled  about  5,000  in  each  case,  the  cost  of 
assaying  alone — apart  altogether  from  the  labor  involved 
in  obtaining  the  samples — amounting  to  an  enormous 
figure.    It  frequently  happens  also  that,  prior  to  an  ex- 

— 170 — 


SAMPLING   AND    ESTIMATION    OF    ORE    IN    A   MINE 


— ijri— 


SAMPLING    AND    KSTIMATION    OF    ORE    IN    A    MINE 

amination,  the  mine  workings  have  to  be  cleared  of  water. 
This  is  an  operation  costly  in  proportion  to  the  quantity 
of  water  to  be  handled.  The  writer  has  had  instances  of 
pumping  amounting  to  over  ii,ooo,  and  a  monthly  cost 
while  the  examination  was  in  progress  of  ^350  for  this 
item  alone. 

The  Sampling  Interval. — In  all  mines  under  the  control 
of  the  writer  the  sampling  interval  is  5  ft.,  and  no  longer 
spacing  is.  in  his  opinion,  to  be  recommended.  The  pre- 
liminary examination  made  by  the  engineer  ought  to  de- 
cide whether  the  mine  is  worthy  of  examination  or  not. 
If  it  be,  then  the  5-ft.  interval  should  be  adopted.  I  have 
found  quite  remarkable  differences  in  sampling  a  length 
of,  say,  500  ft.,  at  lo-ft.  intervals,  and  subsequently  sam- 
pling again  at  intermediate  points,  and  the  difference  as 
shown  by  the  original  10- ft.  sampling  and  the  mean  of  the 
first  and  second  has  often  been  sufficient  to  reverse  the 
conclusions  arrived  at  from  the  10- ft.  intervals.  The 
importance  of  taking  the  samples  at  regular  distances 
cannot  be  over-estimated. 

Sketching  Samples. — The  system  of  sketching  samples, 
as  employed  by  the  writer,  is  illustrated  herewith,  and  is 
based  upon  a  scheme  suggested  by  Mr.  F.  Burnham,  now 
in  the  employ  of  the  State  Mining  Department.  This 
system  gives  in  detail  a  longitudinal  section  of  the  shaft, 
level,  or  winze,  in  a  continuous  length,  and  is  specially 
valuable  when  dealing  with  reefs  having  a  tendency  to 
split.  The  loss  of  any  part  of  the  reef  is  plainly  denoted 
in  the  continuous  section,  and  if  of  sufficient  value  the  lost 
portion  can  be  re-attacked. 

Recording  Samples. — The  recording  of  the  samples  is 
plainly  shown  in  the  accompanying  sheet,  which  is  a  copy 
of  a  sampler's  usual  return  on  the  Meyer  &  Charlton  Mine. 
It  will  be  observed  that  waste  rock  is  always  separately 
shown,  and  in  the  actual  sampling  it  is  never  sectioned, 
but  simply  measured. 

Assay  Plans. — Of  these,  two  are  kept,  one  being  the 
ordinary  outline  development  plan,  and  the  other  a  con- 
tinuous section  plan.     Specimen  extracts  from  the  actual 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 


SAMPLING    AND    ESTIMATION    OF    ORE    I^^    A    MINE 

plan  are  attached  herewith.  The  scheme  of  the  outhne 
plan  is  to  show  both  development  and  stope  values,  the 
width  sampled  appearing  on  the  one  side,  in  the  case  of 
levels,  etc.,  and  the  value  as  upon  the  estimated  stoping 
width  on  the  other ;  the  minimum  stoping  width  for  pur- 
poses of  calculation  being  36  inches.  Referring  to  the 
plan,  it  will  be  seen  that  the  widths  are  shown  in  blue  and 
the  values  in  red.  The  outlines  between  the  levels  show 
the  stope  faces  for  the  various  months,  and  the  assay 
values  are  shown  in  general  monthly  averages. 

If  the  widths  of  reef  fall  below  36  ins.,  the  actual  width 
of  the  reef  is  shown,  but  the  value  is  calculated  always 
upon  a  minimum  of  36  ins.,  and  appears  as  upon  that 
width  opposite  the  actual  reef  width,  the  reason  being 
that  we  never  estimate  the  stoping  width  as  likely  to  be 
less  than  36  ins. 

The  scheme  of  the  continuous  section  plan  is  to  provide 
details  of  each  sectioned  portion  of  the  reef,  and  a  con- 
tinuous actual  sketch  of  the  reef  in  the  levels.  Referring 
to  the  extract,  it  will  be  seen  that  the  sections  sampled  are 
shown  separately,  referring  in  each  instance  to  the  reef 
matter  and  waste  in  their  alternate  bands. 

The  average  recovery  value  of  the  reef  is  shown  on 
continuous  section  plan  by  different  colors  represented  as 

under : 

Per  ton. 

Black  indicates  a  recovery  value  of o  to   20  shillings 

Blue  indicates  a  recovery  value  of 20  to   25      " 

Green  indicates  a  recovery  value  of 25  to   30      " 

Mauve  indicates  a  recovery  value  of 30  to   40      " 

Yellow  indicates  a  recovery  value  of 40  to    50      " 

Violet  indicates  a  recovery  value  of 50  to  100      " 

Gray  indicates  a  recovery  value  of 100  shillings  and  over 

The  recovery  value  (given  in  shillings)  means  the 
actual  estimated  gold  per  ton  which,  from  the  different 
processes,  the  ore  will  yield,  and  is  roughly  based  on  70 
per  cent  of  the  actual  mine  assay  value,  postulating  a 

sorting  basis  of  20  per  cent. 

G.  A.  Denny. 

Johannesburg,  Transvaal,  May  11,  1903. 

(Mr.  Denny  refers  above  to  the  colors  used  to  denote  various  values 
on  the  sampling  diagrams  in  use  at  the  Meyer  &  Charlton  Mine.     The  lim- 

—174— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

ttations  of  our  printing-press  have  compelled  us  to  substitute  in  the  dia- 
grams, cross-hatching  for  colors.  The  explanation  is  given  on  the  dia- 
gram itself,  on  the  page  accompanying. 

In  reference  to  the  sampling  sheet,  given  under  Fig.  i,  it  is  well  to 
add  that  the  total  of  stoping  widths,  in  inches,  viz.,  394,  and  the  total  of 
assay  inches,  viz.,  1527,  is  carried  forward  to  the  end  of  a  series  of  sec- 
tions or  lengths  of  drift  or  stope,  when  the  latter  is  divided  by  the  former, 
which  then  gives  the   average   value   in  dwts.   of  gold  per  ton. — Editob.) 


The  Editor : 

Sir. — The  articles  and  discussions  which  have  appeared 
in  the  Journal  for  the  past  few  months  on  the  sampHng 
and  valuation  of  a  mine  have  been  exceedingly  instruct- 
ive; although  I  have  read  each  week's  contribution  as  it 
came  from  the  press,  yet  within  the  past  few  days  I  have 
read  the  entire  matter  as  a  whole,  and  find  there  is  more 
information  in  such  a  second  reading  than  one  obtains  in 
the  first  perusal  of  the  weekly  editions. 

The  sampling  of  a  mine  is  not  an  easy  or  simple  task,  al- 
though many  self-styled  engineers  think  it  so  and  will 
often  return  from  a  mine  with  a  few  sacks  of  hand-gath- 
ered samples  and  then  write  a  voluminous  report  as  to  the 
merits  of  the  property  and  the  value  of  the  ore-reserves 
therein.  Sampling  is  hard  labor  under  the  best  conditions, 
and  the  actual  moiling  requires  the  skill  of  a  good  miner, 
carefully  watched,  of  course ;  and  I  have  found  that  good 
moiling  is  often  foreign  even  to  miners,  not  to  mention  the 
young  and  uninitiated  who  come  west  to  get  their  first 
experience  in  mining  by  doing  sampling  work.  I  well 
recollect  the  incident  referred  to  by  Mr.  Rickard  in  the  is- 
sue of  February  14,'''  when  he  says,  "It  took  six  men  (three 
of  whom  moiled  while  the  other  three  held  the  boxes  to 
receive  the  sample)  the  whole  of  one  shift  to  take  three 
samples  over  a  vein  12  ft.  in  cross-section,  and  in  accom- 
plishing this  they  dulled  35  moils."  This  incident  was  at 
the  Camp  Bird  mine  some  three  years  ago,  when  it  was 
first  examined.  The  moiling  was  done  by  strong,  able- 
bodied  miners,  and  Mr,  Rickard  put  me  in  charge  of  the 
sampling  at  the  time.  Many  engineers  regard  the  sam- 
pling of  a  mine  as  a  perfunctory  performance,  to  be  gone 
through  lightly,  and  they  aim  to  accomplish  the  task  in  the 
shortest  time  and  with  the  least  trouble. 


Tage   19  of  this  volume. 

-175- 


SAMPLING    AND    ESTIMATION    OF    ORE  JN    A   MINE 

All  these  points  are  well  known  to  the  readers  of  the 
Journal,,  but  I  desire  to  bring  them  out  again,  in  order  to 
more  fully  emphasize  the  hazard  and  risk  courted  when 
the  sampling  of  a  partially  developed  mine  is  given  into  the 
hands  of  an  inexperienced  college  graduate — often  unac- 
companied by  the  engineer  in  charge — ^who  has  come  w  est 
to  supplement  his  college  training  with  actual  practice  in 
the  field.  This  procedure  has  been  adopted  by  some  engi- 
neers in  the  last  two  or  three  years,  but  it  ought  to  be  dis- 
couraged. Unquestionably  it  is  a  cheap  way,  for  the 
young  men  are  very  ready  to  undertake  the  w-ork  and  at  a 
reasonable  figure,  some  of  them  being  glad  to  go  for  their 
expenses  alone.  When  an  engineer  is  retained  to  examine 
a  mine,  his  clients  are  entitled  to  all  the  knowledge  of  that 
property  which  his  years  of  observation  and  experience  are 
able  to  yield  him ;  in  fact,  his  experience,  knowledge  and  in- 
tegrity are  all  that  is  valuable  to  his  clients,  and  he  should 
give  the  products  of  such  qualities  to  them  unremittingly. 
It  is  obvious  that  any  engineer  can  learn  more  concerning 
a  mine  under  examination  by  being  on  the  ground  and 
overseeing  the  actual  sampling,  and  sometimes  taking  a 
few  himself,  than  he  can  possibly  gather  by  going  through 
the  w^orkings  a  dozen  times  and  looking  at  the  places  sam- 
pled after  the  work  is  completed,  especially  in  a  wet  mine 
w^here  the  fresh  cuts  soon  become  smeared.  I  have  known 
of  cases  where  inexperienced  men  have  been  sent  out  to  a 
property  for  the  purpose  of  sampling,  even  before  their 
chief  has  been  on  the  ground,  and  after  the  work  was  com- 
pleted the  engineer  then  came  and  walked  through  the 
workings  and  thought  he  thoroughly  understood  the 
chemical,  physical  and  metallurgical  conditions.  One  en- 
gineer of  my  acquaintance,  and  one  who  is  w^ell-known  in 
this  country  and  abroad,  and  with  whom  I  was  fortunate 
enough  to  be  associated  in  my  days  of  little  experience,  has 
gained  an  enviable  reputation  as  a  thorough  engineer,  and 
I  attribute  his  success,  in  a  great  degree,  to  his  thorough- 
ness in  sampling.  He  never  intrusted  this  work  to  novices, 
and  even  when  employing  an  experienced  man  he  was 
present  invariably  and  superintended  the  work ;  and  to  this 

— t;6— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

day  he  will  be  found  in  his  overalls,  personally  directing 
the  sampling  of  any  mine  which  he  is  examining.  If  one 
is  sick  and  a  surgical  operation  is  necessary,  is  it  satisfac- 
tory, after  calling  in  the  favorite  doctor,  to  have  him  send 
a  fresh  medical  graduate,  whom  he  has  known  but  a  short 
time,  to  perform  the  operation,  and  after  the  wound  is 
dressed  have  the  surgeon  arrive  and  pass  an  opinion  on  the 
case  ? 

Please  understand  that  I  am  not  in  any  way  criticising 
the  employment  of  young  college  men ;  not  at  all ;  in  fact, 
I  am  very  much  in  their  favor,  for  I  was  one  myself ;  but 
I  am  decidedly  against  their  being  sent  to  sample  a  mine, 
without  the  constant  supervision  of  their  chief,  long  be- 
fore they  have  had  sufficient  sampling  experience  or  are 
capable  of  assuming  such  responsibility,  because,  as  before 
stated,  the  sampling  of  a. mine  is  not  a  simple  problem,  and 
I  know  of  no  other  process  in  mine  examination  that  is 
more  important,  or  from  which  the  engineer  himself  can 
gain  more  information.  It  is  neither  fair  to  the  clients  nor 
to  the  young  men.  I  am  glad  to  learn  from  the  discussion 
that  in  several  localities,  especially  in  South  Africa,  the 
engineers  in  charge  recognize  the  importance  of  mine  sam- 
pling to  the  degree  that  the  samplers  are  "culled  largely 
from  trained  engineers." 

Let  me  say,  to  those  who  take  the  trouble  to  transport 
over  the  country  small  hand  rock-breakers  for  reducing 
samples,  that  three  or  four  good  men  with  short-handled 
hammers  and  old  stamp-dies  or  hard  rocks  for  breaking, 
with  a  canvas  underneath,  will  break  more  rock,  and  do  it 
quicker  and  with  less  fines,  than  an  equal  number  of  rock- 
breakers.     I  have  tried  them. 

When  there  are  two  or  more  distinctly  different  vein- 
materials,  or  when  the  vein  does  not  equal  the  stoping 
width,  I  have  found  that  much  better  results  are  obtained 
by  sampling  each  class  of  material  separately,  as  it  is  im- 
possible to  obtain  an  average  by  taking  the  sample  across 
the  entire  width  with  a  continuous  cut,  because  the  differ- 
ent materials  differ  widely  in  specific  gravity.  With  the 
separate  samples  it  is  an  easy  matter  to  calculate  the  aver- 


SAMPLING   AND    ESTIMATION    OF    ORE    IN    A    MINE 

age  across  the  full  width.  This  is  the  best  way  out  of  the 
difficulty  mentioned  by  Mr.  Argall,  in  the  issue  of  June 
13,  in  his  Figs,  i  and  2.f 

I  have  read  the  short  articles  of  ?klessrs.  Richard  Parker 
and  W.  L.  Austin  in  regard  to  the  discrepancy  in  the  as- 
says of  two  quarters  of  a  sample  which  was  rolled  on  can- 
vas. Mr.  Parker  gave  it  as  his  opinion!  that  the  gold, 
which  was  free,  very  easily  penetrated  the  duck,  conse- 
quently the  ore  underwent  a  form  of  concentration.  I  agree 
with  Mr.  Austin§  that  the  gold  does  not  penetrate  the 
canvas ;  at  any  rate,  the  use  of  whisk  brooms  in  quartering 
would  remove  it ;  and  I  am  surprised  that  neither  of  them 
considered  the  fact  that  the  sample  was  wet  or  sticky,  for 
my  experience  is  that  most  any  kind  of  an  assay  can  be 
obtained  from  wet  samples,  especially  if  they  contain  free 
gold.  Without  doubt  the  free  gold  was  enveloped  in  the 
sticky  material  as  the  sample  was  rolled,  and  it  certainly 
could  not  be  homogeneously  mixed  through  the  sticky 
matrix. 

As  to  numbering  samples,  I  have  used  wooden,  brass, 
hard  rubber  and  paper  tags,  and  I  have  found  the  latter  to 
be  the  easiest  and  best.  Use  a  tough  bond  paper  in  sheets 
about  the  size  of  a  nickel  tablet  and  use  a  pigment  pencil. 
Write  the  number  in  the  center  of  the  sheet  and  fold  it  in 
the  center,  then  begin  at  the  folded  end  and  fold  to  the  size 
of  a  lead  pencil  and  crimp  it  by  folding  over  each  end.  In 
this  way  the  number  is  folded  in  many  thicknesses  of 
paper,  and  I  have  found  that  these  withstand  rough  hand- 
ling in  samples  which  are  extremely  wet.  After  the  sample 
is  reduced  and  sacked,  beside  the  number  inside  1  number 
it  by  a  very  small  number,  made  with  a  lead  pencil,  inside 
the  neck  of  the  sack  just  as  close  to  the  string  as  possible. 
It  can  be  found  by  the  sampler,  but  it  is  out  of  sight  anc| 
unknown  to  anyone  else. 

After  trying  canvas  and  boxes  for  catching  the  samples 
I  find  the  box,  a  50-lb.  powder-box,  is  unquestionably  the 


t  On  page  104  of  this  volume. 
t  Vide  Supra  Page    151. 
S    Ante.       Page   152. 


-178- 


SAMPLING    AND    ESTIMATIOX    OF    ORE    IN    A    MINE 

best  thing  to  use.  It  can  be  obtained  readily,  and  can  be 
used  in  wet  levels,  awkward  stopes,  and  anywhere  in  the 
mine.  It  will  catch  all  the  sample  if  held  properly,  and  the 
samples  are  easily  sacked  from  it.  Mr.  W.  H.  Weed  does 
not  explain  how  one  can  use  a  piece  of  canvas,  20  ft.  long 
by  6  ft.  wide,  in  wet  levels,  or  at  the  top  of  high,  narrow 
stopes.*  The  box  merely  requires  the  watching  of  an  area 
of  not  over  4  square  feet,  while  the  latter  necessitates  keep- 
ing an  eye  out  for  dropping  material  from  120  square  feet. 

I  would  like  to  call  your  attention  to  a  method  of  sam- 
pling which  recently  came  to  my  notice.  The  incident  is 
interesting  only  from  the  fact  that  it  shows  what  a  loose 
procedure  may  be  employed  when  an  engineer  is  in  a 
hurry,  due  to  the  pressure  of  business  expediency.  But 
similar  mistaken  methods  have  been  employed  by  others 
who  had  no  such  valid  excuse  and  serious  consequences 
have  ensued  therefrom. 

The  mine  in  question  was  a  mine  of  some  size,  contain- 
ing three  levels  and  having  some  935  ft.  of  workings.  The 
report  of  the  engineer  read  as  follows :  "Cuts  were  made 
across  the  full  width  of  the  vein  from  wall  to  wall  at  inter- 
vals of  three  feet,"  which  should  have  made  some  310  sam- 
ples. Notwithstanding  this  statement,  the  valuation  of  the 
property  was  based  upon  the  assay  value  of  29  samples  as 
located  on  the  section  cut.  Subsequent  investigation  and 
examination  of  the  workings  showed  that  the  statements 
regarding  the  sampling  were  correct,  but  the  results  were 
arrived  at  in  the  following  manner :  Take,  for  instance,  the 
triangular  block  of  ground  between  the  surface  and  levels 
Nos.  2  and  3.  The  sampling  was  begun  in  Tunnel  No.  3 
and  samples  were  taken  every  three  feet,  then  (and  here  is 
the  dumfounding  step)  these  17  samples  were  all  dumped 
into  one  large  sack  and  labeled  "Sample  No.  i."  No  at- 
tention was  paid  to  quantity  of  ore  from  each  three-foot 
section.  As  each  cut  was  made  the  width  was  taken,  and 
the  17  widths  averaged  and  given  as  the  width  for  Sample 
No.  I.    The  same  process  of  mistaken  reasoning  was  fol- 


On  page  145. 


SAMPLING   AND    ESTIMATION    OF    ORE   iN    A   MINE 

lowed  in  obtaining-  samples  2,  3,  19,  18  and  7,  as  well  as 
the  remaining  2^  samples  shown  in  the  cut.  The  samples, 
as  made  tip  from  the  3-foot  cuts,  also  represent  different 
lengths  along  the  drifts.  The  value  of  the  block  was  then 
given  as  follows : 

Length  Total 

No.  Value    Length,  Width,  factor,  Width  foot 

Sample,  per  ton.       feet.        feet.      feet.  factor.  value. 

I $10.86          50          3.7          5  3.7x5=18.5  200.91 

2 7-24          50          2.1           5  10.5  76.02 

3 10.58          40          2.5          4  lo.o  105.80 

19 20.80          60          2.5          6  15.0  312.00 

18 18.34          40          2.5          4  lo.o  183.40 

7 18.82          30          3.0          3  9.0  ,169.38 

27  73-0     $1,047.51 

Average  width,  2.704  ft. 
Average  assay  per  ton,  $14.35. 

The  above  operation  reminds  one  of  that  old  reductio  ad 

absurdum  problem  in  algebra  where,  in  letting  a  =  b,  the 

final  result  proves  the  startling  fact  that  2=1.     The  fal- 

a b 

lacy  was  in  the  assumption  that 7  =  i.     Just  so,  in  the 

above  method  of  sampling,  the  fallacy  was  in  taking  sam- 
ples every  three  feet,  throwing  them  into  one  sack  and  as- 
suming the  sack  to  be  sample  No.  i,  or  the  average  of  the 
50  ft.  of  drift  from  which  they  were  taken.  If  such  a  pro- 
cess of  elimination  gave  the  correct  averages,  it  is  difficult 
to  understand  why  it  was  not  carried  further  and  one  sam- 
ple obtained  for  the  entire  mine,  thereby  reducing  the  assay 
bill  to  $2.50. 

Could  one  have  guessed,  for  it  was  guesswork,  the  value 
of  the  property  after  it  had  been  sampled  by  such  a 
method,  whether  it  was  worth  $500,000  or  $100,000? 
Fortunately  the  mine  was  afterward  sampled  correctly 
and  carefully  at  lo-ft.  intervals  and  in  accordance  with 
the  best  methods,  such  as  have  been  outlined  in  the  articles 
on  this  subject  in  the  Journal,  resulting  in  the  taking  of 
some  150  samples,  and  you  will  readily  believe  that  the 
difference  in  the  gross  value  of  the  ore,  as  arrived  at  from 
the  two  methods  of  sampling,  in  a  property  of  this  size, 
although  containing  only  935  ft.  of  workings,  amounted  to 

—180— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

$95,009.59 !     The  description  of  the  two  methods  affords 
a  sufficient  explanation. 

F.   H.    MiNARD. 

Denver,  Colo.,  August  15,  1903. 


The  Editor: 

Sir. — Owing  to  absence  I  have  but  recently  had  the 
pleasure  of  reading  the  extremely  valuable  articles  on 
"The  Sampling  and  Estimation  of  Ore  in  a  Mine."  I 
trust  this  may  serve  as  an  excuse  for  the  following  rather 
belated  suggestions: 

In  the  consideration,  on  pages  39  to  41,  of  the  much 
discussed  question  of  "High  Assays,"  the  statement 
is  made  that  "the  occasional  barren  and  the  occasional 
high  assay  returns  are  not  comparable  in  their  effect  upon 
the  estimates."  While  this  statement  is  true  in  the  ma- 
jority of  cases,  especially  in  the  sampling  of  gold  mines, 
it  must  be  evident  that  the  effect  of  the  abnormally  high 
or  low  assay  would  depend  entirely  upon  the  ratio  which 
such  assay  has  to  the  average.  Thus,  in  the  series  of 
samples  and  assay  returns,  given  on  page  41,  to  demon- 
strate the  above  statement,  if  all  three  abnormal  results, 
one  very  high  and  two  very  low,  are  omitted,  the  aver- 
age becomes  slightly  less  than  1.25  oz.  per  ton.  The 
abnormally  high  assay,  27.4  oz.,  is  26.15  oz.  higher,  or 
approximately  2,100  per  cent  greater  than  the  average, 
while  in  the  case  of  the  two  low  assays  the  total  of  the 
variation  of  both  from  the  average  is  only  2.5  oz.,  or  200 
per  cent.  Of  course  the  effect  of  these  assays  upon  the 
average  is  increased  or  decreased  when  the  width  covered 
by  the  sample  is  considered,  but  the  above  is  sufficient 
to  illustrate  why  such  abnormal  high  assays  ''affect  the 
estimates  .  .  .  to  an  extent  by  no  means  comparable 
to  the  omission  of  an  equal  number  of  poor  results." 
That  the  occasion  may  arise  wherein  the  elimination  of  a 
low  assay  would  affect  the  result  as  much,  or  more,  than 
the  omission  of  a  high  assay  is  indicated  by  the  follow- 
ing series  based  upon  an  examination  of  a  silver  lead 
property:  ii 


SAMPLING    AND    ESTIMATION    OF    ORE  "iN    A    MINE 

Property:  Oz.  Silver  Foot- 
Number.                                           Width.  per  ton.  Ounces. 

1    5-3  57-4  304.22 

2    4.9  39.2  192.08 

3    4-6  531  244.26 

4    40  59-6  238.40 

5    40  101.3  40520 

6    4-2  54-9  230.58 

7    4.9  42.8  209.72 

8    47  14  6.58 

9    4-5  38.2  171.90 

Total 41. 1  2,002.94 

The  average  of  the  ore,  using  all  of  these  results,  is 
48.7  oz.  per  ton.  If  No.  5  is  omitted,  this  falls  to  43.0 
oz.  per  ton,  while  if  the  low  assay  is  omitted,  the  result 
becomes  54.8  oz.  per  ton.  None  of  these  assays  can  be 
considered  abnormal,  nor  is  the  effect  of  the  elimination 
of  either  very  large,  with  silver  at  53c.  per  oz.  In  this 
particular  instance,  however,  the  eflfect  was  further  em- 
phasized by  the  fact  that  the  sample  containing  101.3  oz. 
silver  per  ton  also  assayed  63.0  per  cent  lead  and  5.5  per 
cent,  zinc,  while  the  sample  containing  low  silver  con- 
tained 3.5  per  cent  lead  and  11.4  zinc,  the  latter  an  espe- 
cially objectionable  feature. 


In  Mr.  Argairs  letter  in  the  issue  of  June  13,  Fig.  2,  he 
shows  what  he  truthfully  states  to  be  "one  of  the  most 
difficult  samples  to  take."     When  tlie   vein  assumes  a 

—182— 


SAMPLING    AND    ESTIMA'JION    OF    ORE    IN    A    MINE 

flatter  dip,  as  in  the  accompanying  figure,  it  becomes 
practically  impossible  to  secure  a  fair  sample  clear  across 
the  vein,  and  it  is  better  to  sample  in  sections,  being  care- 
ful to  get  a  straight  line  across  each  section.  I  find  it  de- 
sirable to  take  samples  very  largely  in  sections  if  there  is 
any  difference  in  the  parts  of  the  vein  which  is  clearly 
observable  by  the  eye.  In  the  examination  of  properties 
not  equipped  with  treatment  plants,  where  the  metal- 
lurgical side  of  the  question  is  necessarily  an  important 
one,  results  so  obtained  are  of  especially  great  assist- 
ance.    Of  course  this  increases  costs. 

The  term  "unequipped  property"  suggests  another 
])oint,  though  perhaps  it  is  beyond  the  pale  of  a  discus- 
sion on  the  sampling  of  a  mine.  Mr.  Denny  remarks 
that  when  the  engineer  is  called  upon  to  decide  whether 
a  ''prospect"  is  valuable  or  otherwise,  and  the  available 
data  are  meagre,  he  is  "forced  to  one  or  two  alternatives, 
either  ( i )  to  intuitively  sum  up  the  value  of  the  property ; 
or  (2)  to  refuse  to  commit  himself  to  an  opinion  until 
more  work  is  done."  Now  some  of  us  who  have  not 
yet  reached  the  Ultima  Thiile  of  being  retained  entirely 
by  corporations  which  purchase  only  "going  properties" 
are  unfortunately,  not  infrequently,  called  upon  by  the 
man  who,  on  the  advice  of  "a  friend  who  has  seen  it  him- 
self," has  put  money  into  a  mine  until  he  is  tired  and 
wishes  to  know  what  he  has  got.  As  Mr.  Denny  states, 
we  are  too  judicious  to  adopt  the  first  of  the  above  alter- 
natives, and  Mr.  Silkmerchant,  who  employs  us,  will  not 
be  satisfied  with  the  second.  It  seems  to  me  there  is  a 
third  possibility,  which  perhaps,  after  all,  is  a  combina- 
tion of  the  other  two.  Why  not  state  to  Mr.  S.  that  the 
present  value  of  the  property  is  only  the  small  amount 
in  cash  which  some  development  company  would  pay,  ad- 
vising from  our  experience  what  this  might  be,  and  admit- 
ting that  from  the  information  obtainable  we  can  not  ex- 
press an  opinion  as  to  probable  future  value.  Tell  him 
it  is  "a  gamble."  By  all  means  let  us  force  upon  the  in- 
vesting public,  if  possible,  the  difference  between  mining 
speculations  and  mining  investments.     Since,   however, 

-183- 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

we  estimate  "possible  ore"  and  "probable  ore"  may  we  not 
state  definitely  and  succinctly,  guardedly,  the  possible  fu- 
ture value  that  might  be  expected  from  the  expenditure 
of  a  stated  sum  in  a  certain  definite  development,  which, 
so  far  as  possible,  is  outlined  in  the  report?  To  be  sure, 
if  this  sum  is  expended  and  the  property  fails  to  come  up 
to  our  guarded  expressions  of  possibilities,  we  are  liable 
to  be  blamed,  especially  by  this  class  of  clients;  but,  as 
Mr.  Rickard  has  said  "difficulty  and  doubt"  have  to  be 
faced,  and  "judgment  and  experience  must  decide." 

Geo.  a.  Packard. 
Baker  City,  Ore.,  July  ii,  1903. 


The  Editor: 

Sir. — I  have  been  much  interested  in  the  discussion  of 
mine  sampling,  and  hope  you  will  continue  to  encourage 
such  exchange  of  views  upon  practical  subjects. 

In  some  of  the  letters  that  you  have  published  it  seems 
that  unnecessary  refinement  and  care  in  doing  the  work 
are  advocated.  For  instance,  I  confess  I  cannot  see  the 
necessity  of  having  a  canvas  20  ft.  in  length,  to  catch 
the  pieces  of  ore  while  sample  is  being  cut.  It  hardly 
seems  advisable  to  cut  such  small  samples  as  to  render  it 
necessary  to  catch  every  minute  particle.  With  a  piece 
of  canvas  6  or  7  ft.  long  and  as  wide  as  the  drift,  using 
also,  when  convenient,  a  box  or  large  Mexican  sombrero, 
good  sampling  can  be  done.  Again,  it  seems  to  involve 
unnecessary  work  and  loss  of  time  to  seal  up  each  sample 
as  taken.  It  is  assumed  that  the  engineer  has  with  him 
at  least  one  man  whom  he  can  trust.  Let  him  also  take 
underground  two  sample  bags,  fitted  with  locks  and  keys — 
one  bag  for  the  originals  and  one  for  the  duplicates.  The 
samples  can  then  be  safely  taken  to  the  surface,  there  to 
be  securely  locked  in  a  box  or  trunk  until  shipped  or  as- 
sayed. If  the  engineer  is  entirely  alone,  then  sealing,  as 
well  as  all  other  imaginable  precautions,  is  necessary ;  but 
it  is  assumed  that  he  will  not  alone  attempt  a  job  of  any 
importance. 

—184- 


SAMPLING   AND    ESTIMATION    OF    ORE    IN    A    MINE 

It  Is  the  custom  of  some  engineers  to  have  the  samples 
(weighing  perhaps  25  lb,  each)  packed  out  to  the  surface, 
to  be  *'bucked"  and  quartered  down.  This  is  a  slow  and 
expensive  method,  unless  the  mine  is  supplied  with  a 
small  crusher,  and  even  then  it  is  better  to  do  a  part  of  the 
''bucking"  and  quartering  underground.  The  engineer 
should  have  two  or  three  careful,  honest  assistants,  to 
watch  and  quarter  samples ;  two  good,  intelligent,  reliable 
miners,  to  cut  the  samples ;  one  man  to  help  in  quartering 
and  tying,  and  three  or  four  to  do  the  bucking  down. 
Such  a  gang  can  do  good  and  fast  work,  and  can  carry  all 
the  tools  and  appliances  necessary  for  the  entire^  operation 
of  cutting,  bucking,  and  quartering  underground. 

I  give  below  a  leaf  from  my  sample  book,  which  is  self- 
explanatory.  It  is  well  to  have  exactly  50  of  such  leaves 
in  each  book,  bound  in  pasteboard.  These  books  possess 
the  following  advantages  :  tags  can  be  numbered  before 
going  down  in  the  mine,  and  numbers  checked;  when 
duplicates  of  each  sample  are  kept,  tags  can  easily  be  torn 
in  two ;  complete  notes  of  each  sample  can  quickly  be  made 
as  sample  is  cut ;  results  of  assays  can  be  recorded  under 
head  of  "Remarks,"  on  the  back  of  the  leaf ;  notes  are  in 
convenient  shape  for  future  reference. 

Sample  Taken 

At  point 

From 

Across 

For Feet Inches. 

Measurement:    Right    Angle    to    Dip — Vertical    or 
Horizontal. 

To 

Dip Strike 

No 

!(over) 

(Perforation  here) 

No 

No 

SAMPLING  CARD. 

-185— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 


—186— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

I  also  include  a  sketch  of  an  iron  ring  and  handle.  Sev- 
eral of  such  rings  add  very  little  to  the  weight  of  one's 
luggage.  When  used  in  conjunction  with  4-lb.  hammers 
and  large  flat  rocks  or  old  stamp-dies,  these  rings  enable 
the  breaking  of  the  ore  (when  reducing  the  bulk  of  the 
sample)  to  be  done  very  quickly  and  thoroughly,  and,  at 
the  same  time,  prevent  pieces  of  ore  from  flying  off  the 
canvas  and  fingers  from  being  smashed.  Sometim.es  it  is 
convenient  to  pack  along  a  small  hand-power  jaw-crush- 
er, but  even  then  these  rings  will  be  found  useful  for  the 
rough  part  of  the  crushing. 

Leather  sample-bags,  two  by  two  by  one-half  foot  in 
size,  made  and  looking  like  mail  bags,  with  leather  strap- 
handle  on  each  end,  are  very  useful.  When  filled  with 
samples,  two  such  bags  make  a  proper  and  convenient 
load  for  a  pack  mule.  Small  pieces  of  tightly  woven  cloth 
are  preferable  to  sample  sacks,  because  suitable  cloth  may 
be  obtained  cheaply  almost  anywhere,  and  the  sample  can 
be  poured  onto  a  cloth  more  easily  than  into  a  small 
sample  sack.  It  is  well  to  mark  the  number  of  the  sample 
on  the  outside  of  each  cloth  with  an  indelible  pencil,  at  the 
time  the  sample  is  tied  up.  For  quartering,  oil-cloth  is 
better  than  canvas  or  muslin,  because  it  prevents  the  fines 
from  sifting  through  and  being  lost  during  the  process  of 
rolling  the  samples.  For  soft  and  moderately  hard  ore,  a 
well-balanced  poll-pick,  made  of  good  steel,  is  the  best  tool 
for  cutting  the  samples;  for  hard  ore,  moils  should  be 
used — for  very  hard  ore,  more  moils.  The  poll-picks 
should  be  made  of  good  drill  steel,  and  should  be  provided 
with  strong  but  springy  handles. 

It  is  the  belief  of  the  writer  that  the  results  of  mine 
sampling  should  be  considered  as  only  close  approxima- 
tions to  the  average  values  of  the  ores  exposed,  and  that 
it  is  hardly  worth  while  to  take  the  trouble  of  making  fine- 
haired  calculations,  to  obtain  average  values.  If  proper 
care  is  used  with  regard  to  abnormal  assays,  the  usual 
method  of  "foot-ounces"  or  "foot-dollars"  gives  results  as 
close  as  are  practical. 

It  is  well  enough  to  talk  and  write  about  taking  a  sam- 

-187— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINK 

pie  that  truly  represents  the  vakie  of  the  ore,  but  doing 
it  is  another  matter.  If  the  ore  for  the  full  width  of  the 
section  cut  is  more  or  less  uniform  as  to  hardness,  and  is 
not  crumbly,  a  good  sample  can  be  obtained ;  but  if  it  is 
decomposed,  porous  and  crumbly,  no  amount  of  care  will 
enable  one  to  cut  a  true  sample.  In  such  cases  it  is  well 
to  take  very  large  samples ;  and,  in  a  general  way,  it  may 
be  said  that  there  is  greater  safety,  not  only  in  large 
samples,  but  also  in  large  numbers  of  them.  If  the  ore 
varies  greatly  as  to  hardness,  the  hard  or  soft  parts  occur- 
ring in  spots  or  bunches  scattered  promiscuously  through- 
out the  vein,  it  is  difficult  to  do  accurate  sampling.  If  the 
hard  or  soft  ore  occurs  in  streaks,  the  difficulty  may  be 
obviated  by  sampling  the  streaks  separately.  Sometimes 
the  only  practical  method  is  to  do  the  sampling  as  care- 
fully as  possible,  realizing  that  it  does  not  truly  represent 
the  ore  values,  and  then,  after  thorough  study  of  the  sub- 
ject, apply  a  percentage  correction  to  the  average  value 
thus  obtained,  as  in  the  four  following  cases : 

Case  I. — Large  bodies  of  low-grade  gold  ore,  in 
which  it  was  suspected  that  the  soft  ore  was  better  than 
the  hard ;  ore-bodies  thoroughly  prospected  in  the  upper 
levels,  and  about  700,000  tons  developed  before  milling 
was  begun ;  ore-bodies  carefully  determined  by  samples 
taken  10  ft.  apart  and  assays  accurately  platted  upon 
maps ;  successful  development  of  ore  was  continued  after 
the  mill  was  started ;  as  a  general  rule,  ore  decreased  in 
value  with  depth.  After  140,000  tons  had  been  milled,  a 
second  examination  of  the  mine  was  made;  new  ore-re- 
serves were  sampled,  and  what  remained  of  the  old  ones 
were  resampled.  Resampling  of  old-bodies  gave  prac- 
tically the  same  results  as  first  sampling.  By  use  of  the 
assay-maps,  stope-maps,  and  records,  a  close  comparison 
could  be  made  between  the  mine  sampling  of  the  140,000 
tons,  and  the  results  from  milling  it,  and  it  was  found  that 
the  mine  samples  should  have  been  discounted  14  2-3  per 
cent.  This  rate  of  discount  was  applied  in  calculating 
the'average  val-ue  of  the  large  tonnage  yet  remaining,  and 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

with  good  results,  as  was  afterward  proved  in  the  milling 
of  it. 

Case  11. — Enormous  body  of  very  low-grade  copper 
ore,  occurring  in  form  of  small  grains  of  chalco- 
cite;  could  be  determined  easily  by  the  eye  that  the  soft 
parts  of  the  ore  were  leaner  than  the  hard;  adits  were 
driven  into  the  ore-body,  and  samples  were  taken  every  5 
ft.,  alternating  from  side  to  side,  so  that  samples  were  10 
ft.  apart  on  each  side;  a  separate  mill-test  was  made  on 
the  ore  from  each  adit;  thus  it  was  possible  to  compare 
the  average  of  the  mine  samples  with  the  average  value  of 
the  ore  as  milled.  Varying  results  were  obtained  in  the 
different  adits ;  in  those  where  bunches  of  soft  ore  were 
scattered  promiscuously  throughout  the  harder  ore,  it  was 
found  necessary  to  add  as  much  as  15  per  cent  to  the  aver- 
age values  shown  by  the  mine  samples,  to  make  them 
correspond  with  the  mill-tests. 

Case  III. — The  property  was  represented  as  being  a 
large  body  of  low-grade,  free-milling  gold  ore,  having  a 
width  of  500  to  600  ft.  The  only  workings  were  two  open- 


jy/e///?  af  Oc^/c/v/>,  j^ooy&  ^00  y^ 


cuts  and  two  shallow  shafts.  The  deposit  had  a  width  of 
outcrop  of  500 to  600  ft.,  but  a  careful  examination  showed 
that  its  actual  thickness,  while  great,  was  only  50  to  60 

—189— 


SAMPLING   AND    ESTIMATION    OF    OKK    IN    A    MINE 

ft.  The  accompanying  sketch  shows  how  the  mistake  in 
estimating  the  width  occurred.  The  ore -deposit  was  in 
quartzite,  containing  here  and  there  small,  irregular  seams 
of  quartz.  It  was  observed  that  the  quartzite  itself  car- 
ried values,  although  values  were  higher  in  the  quartz 
veinlets.  The  gold  was  finely  divided,  and  only  rarely 
visible  to  the  naked  eye.  In  'bucking'  the  samples,  for  the 
purpose  of  quartering  down,  once  in  a  while  a  few  pieces 
of  rock  would  be  found  showing  comparatively  eoarse 
gold,  and  such  pieces  were  rejected  as  a  matter  of  safety. 
The  ore  was  sampled  in  lo-ft.  sections  across  the  deposit, 
269  samples  being  taken.  These  samples  were  'bucked' 
and  quartered  down  by  hand  in  the  usual  manner,  except 
that,  before  the  last  quartering,  they  were  put  through  a 
small  hand-crusher  and  reduced  to  pieces  that  would  all 
pass  /4-in.  mesh.  Samples  were  taken  and  assayed  in 
duplicate,  and  the  average  value  was  $3  per  ton.  The 
price  asked  was  a  very  large  one,  and  as  the  deposit  did 
not  give  much  promise  of  going  down,  the  proposition 
was  rejected.  Later  it  was  bought  for  about  one-fifteenth 
of  the  price  first  asked ;  a  mill  was  built,  and  the  ore  was 
mined  by  the  open-cut  system.  The  writer  was  afterward 
reliably  informed  by  the  manager  of  the  mine  that  the  ore 
as  mined  averaged  about  $3.75  per  ton. 

Case  IV. — In  this  case  the  ore-bodies  were  irregular 
ones  in  limestone,  carrying  gold,  silver  and  lead.  The 
mother  lode  of  the  district  is  a  contact  between  diorite  and 
Cretaceous  limestone.  The  ore  occurs  in  a  system  of  three 
veins,  situated  at  a  distance  of  1,500  to  1,800  ft.  away 
from  the  main  contact.  They  are  parallel  fissure-veins  in 
limestone,  usually  conformable  to  the  strike  and  dip  of  the 
stratification.  The  pay-ore  is  easily  detected  by  the  eye, 
and  in  mining  great  care  is  used  to  keep  it  clean.  There 
was  not  much  ore  *in  sight'  in  the  mine,  but  in  the  lower 
workings  it  had  been  mined  in  such  a  way  that  there  was 
an  opportunity  to  check  the  hand-sampling  of  the  faces 
against  the  smelter  returns  on  product  for  eight  months. 
This  check  gave  the  following  results :  Lead,  as  deter- 
mined by  hand-samples,  practically  the  same  percentage 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

as  was  shown  by  the  smelter  returns ;  silver  values,  from 
hand-samples,  13  per  cent  greater  than  from  smelter  re- 
turns;  gold-values,  from  hand-samples,  36  per  cent  less 
than  from  smelter  returns.  Notwithstanding  these  differ- 
ences in  the  silver  and  gold  values',  it  was  decided  to  rely 
upon  the  results  of  assays  of  hand-samples,  for  the  fol- 
lowing reasons :  The  sampling,  as  well  as  old  smelter  re- 
turns, showed  that  the  ore  was  increasing  in  silver  and 
decreasing  in  gold  values,  with  depth ;  on  the  dump  were 
several  thousand  tons  of  ore  that  had  been  mined  during 
the  two  years  previous  to  the  eight  months  checked,  which 
had  not  been  shipped  on  account  of  lack  of  cheap  trans- 
port facilities ;  this  old  ore  carried  about  the  same  value 
in  lead,  but  was  lower  in  silver  and  higher  in  gold  than 
the  ore  fnined  during  the  eight  months'  period.  This  lat- 
ter ore  had  been  piled  up  against  the  old  ore,  and  in  sack- 
ing it  for  shipment  some  of  the  old  ore  became  mixed 
with  it,  so  that  about  one-fourth-  of  the  ore  sold  to  the 
smelters  during  the  eight  months  was  old  ore. 

One  point  that  I  do  not  remember  having  seen  empha- 
sized in  this  discussion  is:  The  ore  should  be  sampled 
in  the  same  manner  as  it  is  to  be  mined.*  This  is  espe- 
cially true  in  Mexico,  where  labor  is  cheap  and  where 
Mexican  miners  are  very  clever  at  mining  small  streaks 
and  assorting  ore.  The  "plodding  sampler,"  fresh  from 
the  States,  with  no  experience  in  Mexico,  might  go 
through  the  workings  in  an  irregular  lead  deposit  in  lime- 
stone, taking  his  cold-blooded  samples  at  regular  meas- 
ured intervals,  and  then  easily  turn  the  property  down  as 
being  of  no  value;  while,  afterward,  by  using  Mexican 
methods,  modified  somewhat,  perhaps,  by  American  in- 
telligence and  skill,  the  mine  might  be  made  to  pay  thou- 
sands of  dollars. 

Another  practical  point,  that  might  have  been  covered 
to  advantage  by  the  many  engineers  of  wide  experience 
who  have  contributed  to  this  discussion,  is  this :  In  calcu- 
lating ore-reserves,  what  allowances  are  to  be  made  on 


•This  point  was  considered  in  the  original  discussion  of  the  subject;  see 
page  44  of  this  volume. 

—191— 


SAMPLING    AND    ESTIMATION    i)V    <)RF.,1N    A    MINE 

account  of  "horses"  included  within  ore-bodies?  Natu- 
rally, the  percentage  to  be  deducted  varies  \videl\  with 
different  veins  or  deposits,  but  results  in  some  cases  from 
actual  experience  might  be  of  help  in  others.  The  fol- 
lowing is  contributed  as  one  case : 

Conditions:  Large  bodies  of  low-grade  ore  in  an  al- 
tered limestone,  thoroughly  developed :  samples  carefully 
taken  lo  ft.  apart ;  limits  and  thicknesses  of  ore-bodies  de- 
termined by  the  extensive  development  and  sampling; 
average  specific  gravity  determined  by  a  number  of  tests ; 
stope-maps  and  records  during  mining  kept  in  such  a 
manner  that  comparison  could  be  made  between  tons  cal- 
culated and  tons  actually  mined  ;  about  450,000  tons  mined 
by  caving  system  ;  estimated  loss  due  to  caving  system,  not 
over  3  per  cent.  Result :  Many  lime  "horses''  found  with- 
in supposedly  solid  bodies  of  ore,  and  tonnage  mined  was 
20  per  cent  less  than  tonnage  calculated. 

R.  C.  Gem  M ELL. 
Avalos,  Zacatecas,  Mexico,  Sept.  22,  1903. 


The  Editor: 

Sir. — In  reading  reports  on  mining  properties,  I  find  a 
tendency,  when  it  comes  to  the  calculation  of  the  value  of 
the  ore  and  the  profits  in  treatment,  to  mix  up,  unneces- 
sarily, assays  as  expressed  in  percentages,  ounces,  penny- 
weights or  grams,  with  values  and  costs,  as  expressed  in 
dollars,  pounds,  francs  or  pesos. 

I  recently  examined  a  report  on  a  silver  mine  in  Mex- 
ico, in  which  the  assay  maps  purported  to  give  the  value 
of  the  ore  in  United  States  currency.  The  assays  had 
been  made  in  grams  per  metric  ton  of  1,000  kg. ;  this  had 
been  combined  with  the  value  of  silver  taken  at  $35,  Mexi- 
can, per  kilogram,  and  then  exchange  had  been  taken  at 
237.  The  mint  value  of  a  kilogram  of  silver  is  actually 
$40,914,  IMexican  currency,  and  the  figure  $35  only  rep- 
resented the  allowance  from  a  particular  smelter  on  that 
particular  ore,   the   difference  being  supposed  to  cover 


SAMPLING    AND    ESIIMATION    OF    ORE    IN    A    MINE 

metallurgical  losses  and  bullion  taxes.  Exchange  is  con- 
stantly fluctuating  and,  as  nearly  all  expenses  of  mining 
are  based  on  Mexican  currency,'  the  profit  fluctuated  with 
the  rate  of  exchange.  When  it  came  to  determining  the 
comparative  advantages  of  lixiviation  and  smelting,  all 
the  values  in  the  maps  had  to  be  calculated  back  to  grams 
per  metric  ton.  Had  the  maps  shown  this  in  the  begin- 
ning, all  this  trouble  would  have  been  saved.  Nor  should 
these  assays  have  been  reported  in  ounces  troy  per  ton 
of  2,000  lb.  avoirdupois.  It  is  always  well  in  a  foreign 
country  to  figure  and,  if  possible,  think  in  the  standards 
of  the  country,  and  not  make  a  conversion  till  the  very 
last  item  is  reached — the  net  profit  per  annum,  when  the 
amount  may  be  expressed  in  the  currency  of  the  country 
where  the  report  goes. 

It  is  a  very  common  error  in  case  of  a  copper  or  a  lead 
ore  to  express  the  value  in  dollars  by  multiplying  the 
value  of  the  metal  per  pound  by  the  number  of  pounds  of 
metal  shown  in  the  ore  by  assay;  then  at  some  period  in 
the  chain  of  calculation  to  subtract  a  certain  sum  of  money 
to  represent  metallurgical  losses.  When  the  ore  has  to  be 
both  concentrated  and  smelted,  most  unintelligible  figures 
result  and  errors  unwittingly  creep  in,  or  can  be  intro- 
duced by  designing  individuals.  The  only  safe  and  logi- 
cal way  is  to  separate  the  metallurgical  and  commercial 
calculations  into  two  operations. 

As  an  example,  I  give  a  hypothetical  sulphide  copper 
ore  carrying  gold  and  silver,  which  is  first  hand-picked 
and  the  balance  subjected  to  water  concentration,  to  re- 
move an  excess  of  silica.  The  concentrates  are  divided 
into  two  classes,  fines  and  coarse ;  the  fines  are  roasted 
and  then  smelted  in  reverberatory  furnaces,  and  the 
coarse  concentrates,  together  with  the  hand-sorted  ore, 
are  smelted  in  blast-furnaces.  The  resulting  matte  is 
bessemerized  and  shipped  to  the  Atlantic  coast  for  elec- 
trolytic refining.  This  is  an  extreme  case,  but  shows  the 
method  of  calculation,  which  would  be  simpler  in  case 
the  operations  were  fewer. 

We  will  assume  the  treatment  of  1,000  tons  per  day 

—193— 


SAMPLING   AND    ESTIMATION    OF    ORE  iN    A    MINE 

assaying  4  per  cent  copper,  4  oz.  silver  and  0.04  oz.  gold 
per  ton  of  2,000  lb.  avoirdupois.  The  metallurgical  table 
follows : 


Tons, 


1,000        Crude    ore 4 

100        Selected  ore  10 


Assay,  Contents,        Assay,        Contents,       Assay,    Contents 

per  cent  Cu.     lb.  copper,     oz.  Ag.       oz.  silver,      oz  .Au.    oz.  gold. 


900        Concentrating  ore  ....     3.00 
450        Tailings   (a)    1,67 


4SO 


Concentrates    5 


200  Coarse  concentrates  . .      5 

100  Selected  ore  10 

300  B.  F.  charge 

400  B.   F.  slag 0.40 


33.46  Matte  55 


250  Fine  concentrates. .    .  $ 

50        Roasting  loss  (&) 

200        Calcines    6.06 

200        R.  F.  slag 0.70 


23.83  Matte  45 


57.29  Matte 

Converting  loss  (f) . . . 

29        Pig  copper   98.42 

Toll   deductions    (d)  . . 


Refined  copper 


Fine  silver 
Fine  gold   . 


80,000 
20,000 

60,000 
15.000 

4 
10 

3-33 
1.67 

5 

5 
10 

0.20 
54-40 

5 

5.56 
0.30 

48.34 
97.34 

4,000 
1,000 

3,000 
750 

2,250 

1,000 
1,000 

2,000 
80 

1,820 

1.250 
38 

1,212 
60 

1,152 

2,972 
149 

2,823 
141 

0.040 

O.IOO 

0.033 
0.017 

0.050 

0.050 

O.IOO 

0.600 
0.050 

0.063 

0.524 

I.I2I 

40 
10 

30 
7.5 

45.000 

22.5 

20,000 
20,000 

10 
10 

40,000 
3,200 

20 
20 

36,800 

20 

25,000 

750 

12.5 

24.250 
2,800 

12.5 
I2.S 

21,450 

12.5 

58,250 

1. 165 

57,085 

754 

32.5 
32.5 

56,331 

Total  loss 


23,669 


2,682 
1.3 18 


32'S 
7-5 


(o)  Concentration  by  water,  2  into  i,  with  25  per  cent  loss  in  values. 

(b)  Roasting  loss,  20  per  cent  in  .weight,  3  per  cent  of  copper  and  silver. 

(c)  Converting  loss,  2  per  cent  copper,  5  per  cent  silver. 

Id)  Toll  deductions  for  electrolytic  refining  1.3  per  cent  off  electrolytic  assay 
of  pig  copper  and  5  per  cent  off  silver  assay.  Gold  all  allowed  for  at  $30  per  oz. 
troy. 

Coming,  now,  to  the  commercial  side  of  the  problem, 
we  have  the  following  table,  showing  returns  and  ex- 
penditures : 

RETURNS. 

56,331      lb.  copper,  at  13c.  per  lb $7,323-03 

2,682     oz.  silver,  at  55c.  per  oz 1,475.10 

32.5  oz.  gold,  at  $20  per  oz 650.00 


Total    $9,448.13 


— T94- 


SAMPLING    AND    ESTniATION    OF    ORE    IN    A    MINE 
EXPENSES. 

Mining  i,ooo  tons  ore,  at  $1.50  per  ton $1,500.00 

Transport  1,000  tons  ore,  at  25c.  per  ton 250.00 

Administration  1,000  tons  ore,  at  15c.  per  ton 150.00 

Concentrating  900  tons  ore,  at  35c.  per  ton 315.00 

Roasting  250  tons  concentrates,  at  30c.  per  ton 75-00 

B.  F.  smelting  300  tons  ore  and  cone,  at  $2  per  ton 600.00 

R.  F.  smelting  200  tons  calcines,  at  $2.50  per  ton 500.00 

Converting  57,085  lb.  copper,  at  ,006c.  per  lb 342.51 

Freight,  29  tons  copper,  at  $12  per  ton 348.00 

Refining  29  tons  copper,  at  $16  per  ton 464.00 

Comm.,  etc.,  56,331  lb.  copper,  at  .005c.  per  ton 281.66 

Total $4,826.17 

Gross  profit $4,621.96 

From  the  above  figures  the  annual  net  profit  can  be 
deduced  after  allowing  for  a  fixed  annual  amount  to 
cover  construction,  depreciation,  etc. 

I  must  disclaim  any  intention  of  giving  the  above 
figures  of  cost  as  representative,  though  there  are  places 
in  the  West  where  the  conditions  as  to  cost  might  obtain. 
My  idea  is  simply  to  give  a  form  for  calculation  where 
losses  and  costs  are  not  mixed. 

J.  Parke  Channing. 

New  York,  Dec.  14,  1903. 


The  Editor: 

Sir. — As  illustrating  that  mine  sampling,  already  suffi- 
ciently tedious  and  exacting,  can  be  extended  until  it  be- 
comes a  sampling  of  a  sampling,  I  venture  to  contribute 
the  following  notes,  made  in  the  course  of  practical  work 
and  recent  mine  examination  experience;  also  thinking 
that  the  item  may  be  suggestive  without  having  claim  to 
any  great  originality. 

This  note  refers  to  the  sampling  of  the  'quarterings' 
of  a  definite  number  of  samples,  as  convenience  dictates, 
for  subsequent  comparison  of  the  resulting  assay  of  this 
special  sample  with  the  average  assay  results  of  the  par- 
ticular samples  included  in  the  former. 

The  necessity  for  such  practice  as  I  am  about  to  de- 
scribe suggested  itself  to  me  in  an  instance  where  I  was 

—19s— 


SAMPLING    AND    ESTIMATION    OF    ORE   JN    A    MINE 

making  an  examination,  and  had  to  limit  myself  to  one 
assistant,  under  circumstances  which  demanded  more 
than  usual  precaution  in  the  care  to  be  exercised  in  the 
sampling,  as  well  as  in  the  safeguarding  of  the  samples 
taken. 

A  pre-requisite  to  the  carrying  out  of  this  scheme  of 
sampling  check  is  that  the  sampling-cut  shall  be  made  of 
a  regular  width,  so  that  the  wider  vein  shall  be  repre- 
sented in  its  due  proportion,  as  also  the  narrower  (and 
generally  higher  grade)  streak  in  its  proportion. 

The  sampling  described  herein  was  more  extensive 
than  the  sampling  numbers  indicate,  but  the  part  of  it 
published  will,  I  think,  sufficiently  illustrate  the  idea 
which  it  is  wished  to  convey.  The  quartering  or  reduc- 
ing of  the  samples  was  done  in  the  mine  as  sampling  pro- 
gressed ;  the  calculation  of  averages  is  based  on  the 
geometric  mean  or  "foot-dollar"  method.  The  detail  of 
the  sampling  compares  with  the  result. of  special  samples 
as  follows : 


No. 
II 

12 
13 
U 
15 
16 


Vein 
widthj 
ft. 

4 

5-5 
2 

3-5 

5 

3 


Assay  value 
$3.60^ 
500 
5.00 
0.60 
3.60 
1.20 


(Special  sample  of  quarterings  of 
Sampleji  11  to  16,  inclusive,  $2.70. 


3  5/6  ft.  av.     $4.10 


20 
21 
22 
23 


4-5 
6.5 

5-5 
5-5 


$3.20 
2.60 
8.00 

3-20 


1/3  ft.  av.      $4.13 


Special  sample  of  quarterings  of 
'Samples  20  to  23,  inclusive,  $3.20. 


26 

5-5 

$3-60. 

27 

7 

1.80 

28 

6 

3.20 

29 

6 

1.80 

30 

5-5 

4.00 

32 

6 

*i8.oo 

6  ft.  av 

$2.70 

Special  ."sample  of  quarterings  of 
Samples  26  to  32.  inclusive,  $1.50. 


t96— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 


42 

2.5 

$8.00 

43 

3.5 

6.80 

44 

3 

0.40 

45 

3 

2.40 

46 

3 

2.20 

47 

3.50 

1.60 

48 

3 

0.60 

49 

5 

0.40 

3  1/3  ft.  av.      $2.60 


Special   sample  of  quarterings  of 
Samples  42  to  49,  inclusive,  $1.60. 


51 

3 

$4.80 

52 

4 

6.80 

S3 

4 

1.40 

54 

3 

1.40 

3-5  ft.  av. 

U67. 

^Special  sample  of  quarterings  of 
Samples  51  to  54,  inclusive,  $3.10. 


Original, 

oz.  gold 

Special  sample. 

per  ton. 

II  to  16 

0.14 

20  to  23 

0.16 

26  to  32 

0.08 

42  to  49 

0.08 

51  to  54 

0.18 

*Eliminated  sample  No.  22  as  ''spotty"  and  substituted  average 
value. 

The  care  taken  in  preparing  these  samples  and  the  ac- 
curacy of  the  assaying  is  indicated  by  the  uniformly  close 
result  of  the  control  assay,  which  was  taken  to  a  public 
assayer  of  good  repute.     Thus  : 

Control  assay, 
oz.  gold 
per  ton. 
0.13 
0.16 
0.07 
0.08 
0.13 

While  this  particular  ore  happens  to  be  a  low-grade 
ore,  it  might  be  interesting  to  carry  out  a  similar  test  on 
ores  of  higher  grade  and  different  character. 

It  will  be  noticed  that  the  special  sample  of  the  26  to  32 
series  catches  the  irregularity  occurring  in  No.  32  sam- 
ple, and  proves  that  the  nature  of  this  discrepancy  was  in 
the  assay  rather  than  in  the  sample  itself. 

Another  thing,  the  result  of  the  quarterings  sample  is, 
in  every  case,  appreciably  lower  than  that  of  the  average 
of  the  equivalent  sample  series.  As  these  samples  were 
all  of  them  quite  dry  when  quartered  down,  this  latter  ob- 
servation rather  indicates  that  the  lines  which  cling  to 
canvas  to  some  extent  (and  almost  unavoidably)  tend  to 
carry    higher   values    than    the    coarser   material.     This 


SAMPLING   AND    ESTIMATION    OF   OREMN    A    MINE 

property  of  canvas  has  been  remarked  in  other  papers  on 
the  subject  of  sampHng;  and  it  is  my  observation  that  the 
extent  to  which  the  fine  residuum  or  dust  that  chngs  to 
canvas,  and  with  it,  sometimes,  particles  of  very  fine  me- 
tallic gold,  can  be  approximately  determined  in  almost 
any  case.  This  can  be  done  by  the  smiple  resort  of  tak- 
ing, and  repeatedly  taking,  about  an  even  bulk  (relative 
to  samples)  of  absolute  waste  material,  and  reducing  this 
and  quartering  it  down  in  the  same  way  as  the  regular 
samples  of  '.he  mine.  In  addition  to  the  special  samples, 
I  took  precaution  to  include  among  my  samples  a  number 
of  waste  samples.  Such  samples  constitute  not  only  a 
check  on  the  sampling,  but  on  the  assayer,  and  bring  to 
light  any  irregularities  that  might  possibly  creep  into  the 
most  careful  sampling.  I  am  inclined  to  think  that  there 
is  much  more  said  about  the  inclusion  of  waste  samples 
than  is  actually  carried  into  practice. 

While  the  extra  trouble  of  sampling  the  discarded  por- 
tions of  regular  mine  sample  may  not  always  be  war- 
ranted, nor  oftentimes  be  deemed  necessary,  it  certainly 
gives  additional  data  upon  which  to  depend  for  informa- 
tion of  general  values,  and  under  some  circumstances  it 
more  than  compensates  for  the  trouble  that  it  involves. 

Forbes  Rickard. 
Denver,  Colo.,  Dec.  i,  1903. 


—198— 


REVIEW  OF  DISCUSSION 


By  T.  a.  Rickard. 

In  closing  the  discussion  on  mine  sampling,  I  desire  to 
thank  those  who  have  contributed  their  comment  and  ex- 
perience. Whatever  service  my  treatment  of  the  subject 
may  have  done,  it  has  been  much  increased  by  the  dis- 
cussion. 

The  efforts  to  give  precision  to  the  terms  used  in  de- 
scribing the  ore  opened  up  by  mining  operations  will,  I 
doubt  not,  leave  their  influence  on  the  practice  of  the  pro- 
fession. Already  "ore  in  sight"  is  tabooed  and  "ore  de- 
veloped" has  appeared  in  several  reports  upon  mines.  Mr. 
Argall  has  driven  the  last  nail  in  the  coffin  of  a  phrase 
which  was  previously  almost  in  extremis  by  reason  of  the 
action  of  the  Institution  of  Mining  and  Metallurgy,  whose 
circular  (published  in  the  Engineering  and  Mining 
Journal  of  October  i8,  1902)  made  a  distinct  pro- 
nouncement concerning  the  careless  use  of  such  terms. 
In  endeavoring  to  supply  a  definition  the  council  of  the 
Institution  did  a  good  service  to  engineering  practice,  but 
ventured  upon  dangerous  ground.  No  set  terms  can  al- 
ways cover  the  relative  degree  of  evidence  to  be  obtained 
concerning  the  tonnage  of  ore  available  in  different  mines. 
Each  mine  has  its  own  "personal  equation,"  as  it  were; 
in  each  instance  there  is  a  degree  of  regularity,  or  irregu- 
larity as  it  may  be,  of  ore  occurrence,  both  as  regards 
value  and  width,  so  that  two  sides  of  a  block  as  exposed 
in  one  class  of  mine  possessing  persistent  ore-shoots  af- 
fords better  assurance  of  a  certain  amount  of  ore  in  re- 
serve than  four  sides  in  other  mines  where  the  vein  is 
sporadic  and  fickle.  This  point  has  been  brought  out  by 
—199 — 


SAMPI.lNd    AND    ESTIMATION    OF    ORE  WN    A    MINE 

Messrs.  Geo.  E.  Collins,  Chester  W.  Pnrington  and  John 
C.  Treadwell.^ 

This  aspect  of  the  enquiry  is  also  emphasized  by  the 
consideration  of  unsystematic  sampling,  as  outlined  by 
Mr.  Geo.  J.  Bancroft.-  The  writer  happens  to  know  the 
two  engineers  whose  methods  are  adumbrated  by  Mr. 
Bancroft,  and  he  can  appreciate  that  men  of  such  wide  ex- 
perience are  able,  by  the  exercise  of  sagacity,  to  make  one 
or  two,  apparently  haphazard,  tests  which  will  throw. more 
light  on  the  character  of  a  mine  than  the  laborious  sam- 
pling of  an  inexperienced  man.  Short  cuts  are  proverbi- 
ally dangerous  to  the  novice,  while  they  may  be  safe  to  a 
veteran.  There  is  no  doubt  that  clients  who  have  to  pay 
heavily  for  the  detailed  sampling  of  a  large  mine  will  ap- 
preciate the  acumen  of  an  engineer  who  by  a  few  tests, 
judiciously  distributed,  can,  at  a  small  expense,  find  out 
the  real  inwardness  of  things.  This  applies  particularly 
to  preliminary  examinations. 

The  shape  of  ore-bodies  is  an  important  factor,  as  Mr. 
Purington  makes  clear."  One  cannot  sample  intelligently, 
that  is,  one  cannot  arrive  at  the  size  and  average  composi- 
tion of  a  large  mass  of  any  material  unless  one  has  some 
notion  of  the  shape  of  it.  Here  geological  structure  plays 
an  important  part,  as  it  has  been  my  endeavor  to  show  in 
the  two  or  three  instances  which  I  have  described.  The 
instances  have  been  invariably  founded  on  fact  and  de- 
scribed with  care,  so  that  many  readers  must  have  been 
able  to  spot  the  localities  from  which  they  were  borrowed. 

That  sampling  is  hard  work,  as  well  as  an  important 
part  of  mining  practice,  is  a  fact  scarcely  needing  empha- 
sis to  professional  men,  but  I  doubt  if  it  is,  as  yet,  proper- 
ly appreciated  by  the  people  in  whose  interests  it  is  done. 
Therefore  the  detailed  description  of  the  sampling  of  the 
bottom  of  a  wet  level  by  Mr.  Ernest  Levy  will  do  good 
service.*     Moreover,  it  suggests  the  extreme  care  required 


^See  pages  93,  96  and  115. 
2Page  118. 
'See  page  96. 
*See  page  124. 


SAMPLING    AND    E.STI>:AII0N    OF    ORK    IN    A    MINE 

to  sample  underfoot,  especially  in  a  wet  mine.  As  a  rule, 
if  suflficient  data  are  otherwise  procurable,  it  is  best  to 
leave  the  bottoms  of  levels  alone,  because  of  the  great 
danger  of  vitiated  results  due  to  the  washing  of  fine  ore 
into  the  channeling  made  by  the  moil  or  pick;  moreover, 
it  is  rarely  possible  to  be  sure  that  the  proper  width  of  ore 
is  being  broken  when  working  in  the  bottom  of  a  drift. 
The  necessity  to  tear  up  the  track,  stop  the  tramming  and 
interfere  with  the  operations  in  any  part  of  the  mine 
causes  the  management  to  look  with  disfavor  upon  the 
sampling  of  bottoms  and  leads  usually  to  the  work  being 
done  in  a  hurry,  under  great  discomfort  and  under  serious 
disadvantage.  These  tend  to  impair  the  reliability  of  the 
results.  Therefore,  if  practicable,  sample  the  backs  over 
the  level  or  the  nearest  winzes  in  preference.  If  these  are 
not  available,  that  is,  if  the  ground  above  the  level  is 
stoped  out  and  the  ground  underfoot  has  not  been  pene- 
trated by  winzes,  then  sample  underfoot  and  make  up 
your  mind  not  to  be  rushed,  to  do  it  carefully  and  to  have 
the  operation  conducted  under  your  immediate  super- 
vision. 

Mr.  Chester  F.  Lee  asks^  whether  it  is  admissible  to 
combine  several  cuts  across  the  vein  at  short  intervals  or 
to  take  separate  samples  at  greater  intervals?  To  this  I 
would  say  that  the  mixing  of  samples  obscures  the  in- 
formation which  they  can  give  and  spreads  the  element  of 
error.  If  one  is  limited,  by  time  or  money,  in  the  number 
of  samples  to  be  taken,  I  would  prefer,  for  example,  one 
hundred  separate  cuts  at  intervals  of  ten  feet  to  the  same 
number  of  samples  obtained  by  combining  every  two  cuts 
taken  at  intervals  of  five  feet.  If  a  bad  sample  is  taken, 
either  by  encountering  an  accidental  rich  spot  or  by  break- 
ing an  undue  proportion  of  one  part  of  the  cross-section 
of  the  vein,  then  by  mixing  this  with  the  next  cut,  the 
error  is  only  made  so  much  more  difficult  of  detection. 
Moreover,  such  sampling  by  requiring  twice  the  number 
of  cuts,  without  any  increase  of  precision,  unnecessarily 


•See  page  129;  also  Mr.  Gemmell's  remarks  on  page  184. 

— 201  — 


SAMPLING    AND    ESTIMATION    OF    ORI?   IN    A    MINE 

augments  the  time  and  labor  involved.  Each  assay  re- 
sult and  each  width  represents  a  factor  in  the  final  calcu- 
lations for  tonnage  and  value ;  the  more  of  these  there  are 
the  closer  the  estimate.  But  by  combining  two  adjacent 
cuts  there  is  no  increase  in  the  number  of  assay  results, 
although  there  may  be  double  the  number  of  measure- 
ments for  width.  In  an  erratic  ore-streak  the  latter  would 
prove  useful. 

Mr.  T.  Lane  Carter  and  Mr.  G.  A.  Denny  have  enriched 
the  discussion  by  their  descriptions  of  South  African 
practice.®  Nowhere  in  the  world  is  sampling  so  nearly  an 
exact  science  as  in  the  mines  of  the  Transvaal,  simply 
because  these  contain  ore-deposits  of  such  persistence  and 
uniformity  that  they  afford  the  basis  for  careful  work. 
The  stuff  is  there,  and  it  is  not  so  erratic  in  occurrence  or 
in  richness  as  to  upset  calculations  founded  upon  the  doc- 
trine of  averages.  After  all,  the  value  of  any  system 
must  depend  upon  the  comparative  regularity  of  condi- 
tions assumed  to  exist  by  the  mere  fact  of  a  regularity  of 
method.  Stereotyped  procedure  is  stultified  by  the  want 
of  some  measure  of  uniformity.  For  these  reasons,  the 
engineers  on  the  Rand  have  had  a  great  opportunity  to 
improve  sampling  methods.  This  they  have  abundantly 
done.  One  of  the  best  systems  is  the  scheme  of  a  con- 
tinuous section-plan,  proposed  by  Mr.  F.  Burnham,*  which 
has  been  adopted  by  several  mines,  and  is  described  by 
Mr.  Denny.  By  indicating  the  average  assay  of  separate 
lengths  of  reef  by  means  of  tints  upon  the  mine  map,  it  is 
possible  to  get,  at  a  glance,  the  relative  value  of  the  work- 
ings ;  the  unit  is  a  full  stoping  width,  so  that  the  tints 
mean  not  the  gold  contents  per  ton  of  any  streak  or  vein, 
but  the  value  as  sloped — a  very  different  thing.  This 
continuous  section-plan  is  of  great  service  to  the  manage- 
ment of  a  large  mine.  When  supplemented  by  the  sam- 
pling record,  which  contains  a  continuous  sketch  of  the 
distribution  of  the  pay-streaks  in  a  wide  vein,  as  shown  in 


•See  pages  131   and   163. 

•'Continuous  Section  System  Mine  Sampling,'  by  ^I.  H.  Burnham,  Trans- 
actions Institution  of  Mining  and  Metallurgy,  Vol.  X,  pp.  204-218. 


— 202 — 


SAMPLIXr,    AND    ESTIMATION    OF    ORE    IN    A    MINE 

the  section-plan,  this  system  affords,  in  a  compact  and 
logical  manner,  a  great  deal  of  the  vital  information  neces- 
sary to  good  management. 

Mr.  Denny  pleads  for  a  minimum  of  5  ft.  as  the  sam- 
pling interval,  and  quotes  differences  observed  when  a 
longer  interval  has  been  adopted.  It  is  obvious  that  the 
shorter  the  interval  the  more  accurate  the  averages  ob- 
tained, for  every  interval  represents  a  column  of  ore  hav- 
ing the  length  of  the  interval,  the  width  of  the  ore,  and 
the  height  of  the  block.  The  last  dimension  is  shared, 
usually,  with  a  corresponding  sample  on  the  next  level. 
In  practice  the  interval  is  usually  determined  by  the  exi- 
gencies of  time  and  expense.  When  a  mine  is  well  or- 
ganized and  has  men  who  are  told  off  to  attend  regularly 
to  the  sampling  of  the  workings,  it  is  practicable  to  reduce 
the  interval  and  do  the  work  up  to  the  extreme  of  detail ; 
when,  on  the  contrary,  an  engineer  is  sampling  a  mine 
preparatory  to  submitting  a  report  upon  it,  he  is  apt  to  be 
limited  by  time  and  by  the  funds  placed  at  his  disposal  by 
his  client ;  as  a  consequence,  he  cannot  spend  all  summer 
on  one  examination.  It  depends  as  much  upon  the  client 
as  the  engineer;  the  better  class  of  the  former  will  not 
hurry  the  work  and  will  not  stint  the  expenditure  neces- 
sary to  do  it  well.  One  can  only  say  that  the  engineer 
cannot  be  too  careful,  and  to  that  end  he  will  endeavor  to 
find  time  and  money  sufficient  to  take  samples  at  the 
shortest  interval  required  by  the  nature  of  the  lode.  In 
one  mine,  quoted  by  me,  I  sampled  every  three  feet,  and 
even  then  the  interval  was  short  enough,  seeing  that  the 
vein  averaged  4  inches  of  ore  carrying  500  oz.  of  silver 
per  ton;  in  other  cases,  of  uniform  low-grade  pyritic 
lodes,  a  20-ft.  interval  would  give  closer  results  than  the 
3-ft.  interval  in  the  case  just  referred  to. 

Mr.  Denny  pleads  for  the  general  expression  of  values 
in  terms  of  currency.  This  is  a  good  point.  Ounces  of 
gold  mean  bullion  of  uncertain  value;  ounces  of  silver, 
percentages  of  copper,  lead,  etc.,  vary  in  value  with 
changes  in  the  market.  Mining  is  a  matter  of  money.  I 
remember  hearing  of  the  case  of  an  engineer  who  exam- 

— 203 — 


SAMPLING    AND    ESTIMATION    OF    ORE;  IN    A    MINE 

incd  an  Indian  mine  in  the  early  days  of  the  ^^'ynaad,  and 
instead  of  using  the  assay-ton  he  employed  the  old-fash- 
ioned percentage  method,  with  the  result  that  he  mis- 
placed the  decimal  point!  A  colossal  blunder  followed. 
If  he  had  used  currency  instead  of  percentages  as  his 
unit,  he  would  not  have  made  such  a  fatal  slip.  Further- 
more, mining  operations  nowadays  cover  regions  of  such 
diversity  that  the  bullion  contents  may  range  from  a  50 
per  cent  alloy  to  gold  of  995  fineness.  To  report  in  quan- 
tity of  bullion  is  only  to  afford  one  more  chance  of  mis- 
understanding, and,  after  all,  the  purpose  of  all  engineer- 
ing work  is  to  get  at  the  facts  and  then  to  state  them  be- 
yond the  peradventure  of  misconception. 

Mr.  T.  Lane  Carter  describes"  current  practice  on  the 
Rand  and  the  arrangements  made  by  the  mine  manage- 
ment for  the  regular  sampling  of  the  workings.  It  is 
plain  that  at  Johannesburg  the  sampler  has  won  a  distinct 
position  for  himself.  Incidentally  he  accentuates  the  use- 
fulness of  correct  assay-plans  as  indicating  the  value  of 
any  portion  of  the  reef  which  is  too  poor  to  be  worked 
now,  but  may  be  found  profitable  under  more  favorable 
economic  conditions.  When  that  time  comes,  an  accurate 
record  of  the  values  as  determined  by  the  previous  daily 
sampling  of  the  drifts  will  be  found  of  great  service.  The 
use  of  surveyor's  points  to  locate  the  places  sampled  is  a 
good  scheme,  since  other  points  of  departure  may  become 
obscured.  Furthermore,  it  suggests  the  friendly  relations 
which  should  exist  with  the  surveyor's  department ;  in 
fact,  it  would  be  well  in  most  mines  to  put  the  sampling 
work,  and  the  records  of  it,  under  the  charge  of  the  sur- 
veyor attached  to  the  property. 

Mine  sampling  on  the  Rand  has  been  developed  to  the 
dignity  of  a  distinct  branch  of  practice;  therefore,  Mr. 
Denny's  description  of  the  methods  in  vogue  is  most  op- 
portune. His  criticism®  in  regard  to  the  ascertainment  of 
the  "average  value  of  the  ore  in  the  past"  from  "the  rec- 
ords of  a  mine"  is  due  to  a  misconception  of  my  meaning. 


'Pages   131   to   138. 
*On   page    1 66. 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

I  meant  the  value  per  ton,  as  treated  in  the  mill  or 
smelter,  of  the  ore  produced  in  the  past,  and  not  the  aver- 
age yield  of  the  ground  which  has  been  removed  by  stop- 
ing.  I  quite  agree  with  Mr.  Denny  that  it  is  difficult  to 
arrive  at  the  yield  per  fathom,  or  per  square  foot,  of  lode 
in  workings  which  have  been  abandoned ;  and,  of  course, 
we  are  well  aware  that  many  a  false  notion  of  the  grade 
of  lode-contents  has  arisen  from  a  literal  acceptation  of 
the  story  told  by  mill  and  smelter  records.  On  the  other 
hand,  if  the  examination  of  a  mine  extends  over  several 
months,  it  is  occasionally  possible  to  check  sampling  re- 
sults by  measuring  a  block  of  ground  and  then  getting  the 
actual  yield  of  it  in  tons  of  ore  and  in  ounces  of  bullion. 
There  are  also  instances  where  the  management  of  a  mine 
is  in  honest  hands — for  all  sellers  of  mines  are  not  dishon- 
est, nor  are  all  buyers  immaculate — and  then  it  may  hap- 
pen that  the  stope-maps  have  been  kept  in  such  a  way  that 
the  yield  per  fathom,  the  average  width  of  clean  ore  as 
compared  to  the  stoping  width,  the  percentage  of  sorting, 
and  other  data,  can  be  secured  and  utilized  as  a  check  on 
the  estimates  based  on  sampling  alone. 

Mr.  J,  H.  Curie  taps  the  milk  in  the  cocoanut  in  his 
contribution  on  mine  valuation.''  It  is  the  net  profit  tliat 
an  engineer  is  asked  to  determine ;  all  the  rest  is  noise  and 
smoke;  tonnage  of  ore,  assa3^s,  machinery,  water  power, 
timber  rights,  dump  facilities,  scenery — all  these  are  mere 
frills  on  the  outside  of  the  essential  problem.  How  much 
money  will  the  mine  yield?  This  is  being  recognized 
more  and  more ;  twenty  years  ago  it  was  customary  to 
make  a  perfunctory  trip  through  the  underground  work- 
ings, to  break  off  a  specimen  or  two,  and  then  to  spend  a 
few  days  copying  the  past  records  of  production,  getting 
tracings  of  the  maps  and  interviewing  the  oldest  inhab- 
itants. With  such  data,  it  is  no  wonder  that  mine  reports 
became  a  thing  for  derision.  It  is  just  such  plain  talk, 
straight  from  the  shoulder,  as  this  of  Mr.  Curie's,  which 
goes  to  the  heart  of  the  problem  and  brings  the  thoughtful 


•See  pages   138  to   145. 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

engineer  into  touch  with  the  financial  side  of  a  great  in- 
dustry ;  it  means  the  passing  of  the  professor  type  and  the 
development  of  the  experienced  engineer,  who  knows 
human  nature  and  the  brutalities  of  the  financial  arena  no 
less  than  the  contents  of  text-books  and  the  talk  of  the 
library. 

There  is  no  doubt  that  Mr.  Curie  is  right  in  saying  that 
the  environment  of  the  English  engineer  is  against  him  in 
this  respect.  In  America  the  engineer  deals  directly  with 
a  capitalist  who  usually  knows  a  bit  about  the  business  of 
mining,  but  in  London  he  is  apt  to  run  against  a  wholly 
impossible  type,  the  middleman,  to  whom  the  mine  is 
merely  the  basis  for  clever  dealing,  or  the  titled  incompe- 
tent who  is  in  the  hands  of  the  former  and  plays  a  part 
like  that  of  the  Indian  whose  wooden  image  stands  in 
front  of  the  cigar  store. 

Mr.  Curie  speaks  of  a  mine  valuer  taking  certain  risks 
in  figuring  out  the  ore  reserves ;  it  is  not  in  the  estimates 
of  ore  blocked  out  that  the  chances  of  error  are  so  great 
as  in  appraising  the  future  prospects,  the  "good-will,"  as 
it  were,  of  the  property,  that  the  largest  margin  for  differ- 
ence will  occur.  Mines  are  not  bought  only  for  the  ore 
actually  calculable  in  the  existing  workings,  but  also  with 
the  idea  of  a  future  enhancement  in  value  due  to  intelli- 
gent development.  Here  it  is  where  a  man  must  be  true 
to  himself  and  to  his  employers,  by  facing  the  facts  frank- 
ly; and  if  the  facts  are  insufficient  to  warrant  a  forecast, 
let  him  dig  them  out  by  further  investigation.  If  they 
are  not  available  after  thorough  examination  of  all  the 
circumstances,  then  let  the  engineer  say  so  frankly,  to  the 
end  that  his  client  may  know  just  what  risk  he  is  taking; 
in  many  cases  a  risk  may  be  warranted,  in  others  the 
condition  of  the  mine  and  its  past  history  may  render  it 
out  of  the  question. 

Mr.  Argall  emphasizes^^  the  necessity  for  taking  samples 
of  approximately  equal  weight;  this  is  important.  Of 
course,  the  samples  will  vary  in  bulk  with  the  width  of  the 
lode  ;  the  amount  broken  should  be  equal  per  foot  of  cross- 

i^On  page  103. 

— 206 — 


SAMPLINCr    AND    KS'l  I.M  A'l'lOX    C)F    ORE    IN    A    MINE 

section.  When  you  leave  it  to  such  accidental  factors  as 
hardness  and  accessibility  to  fix  the  size  of  the  sample, 
you  simply  introduce  an  unnecessary  element  of  error, 
because  excess  of  ore  broken  means  usually  that  it  comes 
from  a  soft  part  of  the  lode,  while  a  meager  sample  will 
coincide  with  extra  hardness.  In  most  lodes  the  differ- 
ence in  ease  of  fracture  is  due  to  some  constituent  min- 
eral, the  presence  of  which  is  likely  to  coincide  with  a 
variation  in  the  value  of  the  ore. 

It  is  good  practice  to  brush  the  face  clear  of  dust  or  re- 
move any  mud  which  is  not  in  place,  because  this  foreign 
material  frequently  contains  finely  powdered  sulphides 
which  may  vitiate  the  sample,  especially  in  a  low-grade 
gold  ore.  Mr.  Argall  makes  several  other  good  sugges- 
tions, such  as  that  of  allowing  two  gangs  of  samplers  to 
take  alternate  intervals  in  sampling,  thereby  checking  each 
other's  work.^^  As  to  the  use  of  a  hand  rock-breaker,  that 
is  almost  imperative  where  a  big  mine  is  being  investi- 
gated; it  will  facilitate  the  careful  quartering  down  of 
samples. 

Both  Mr.  Curie  and  Mr.  Argall  discuss  that  much- 
mooted  question :  what  to  do  with  erratic  high  assays.^ - 
This  vv^as  considered  at  some  length  in  my  own  treatment 
of  the  subject  ;^"  but  as  a  general  method,  to  be  departed 
from  only  where  peculiar  conditions  arise,  as  in  the  cases 
described  by  Mr.  Curie  and  by  myself,  I  do  not  know  of 
any  better  advice  than  that  of  Mr.  Argall,  that  is,  to  reject 
the  erratic  results — and  there  will  not  be  many  of  them, 
otherwise  they  are  not  erratic — and  then  insert  the  aver- 
age values  as  given  by  the  other  results.  In  getting  at 
this  average  it  will  be  well,  it  seems  to  me,  to  omit  those 
results  which  give  nil  and  insert  the  figure  given  by  aver- 
aging only  the  pay  ore.  This  will  lean  a  little  to  the  idea, 
which  is  true,  that  these  erratic  assays  do  indicate  unusu- 
ally rich  spots — but  spots,  not  masses. 


"See  page  105. 

i-See  pages  in  and  142.    Also  Mr.  Packard's  remarks  on  low  assays,  given 
on  page   i8t. 

i"On  pages  39  to  44. 

— 207 — 


SAMPLING    AND    ESTIMATION    OF    ORE   IN    A    MINE 

Thus,  take  the  following  list  of  actual  assay  returns 

from  the  sampling  of  a  gold  mine : 

No.  of  Width       Gold  value  in  dollars Foot- 
Sample,  in  feet  Original.  Duplicate.  ISIean.  dollars. 

1 7.0  $24.20  $48.00  $36.10  $252.70 

2 4.0  10.80  8.00  9.40  37.60 

3 12.0  24.00  23.20  23.60  283.20 

4 8.0  84.00  78.00  81.00  648.00 

5 5.3  740-00  496.00  618.00  3,27540 

6 7.0  32.00  40.00  36.00  252.00 

7 3.5  19.20  64.00  41.60  145.60 

8 6.0  8.00  8.40  8.20  49.20 

9 6.0  24.40  64.00  44.20  265.20 

ID 7.0  28.00  16.00  22.00  154.00 

II T.z  960  40.00  24.80  181.04 

12 8.0  230.00  392.00  311.00  2,488.00 

13 2>'Z  30.40  45.60  38.00  125.40 

14 9.8  6.60  15.20  10.90  106.82 

IS 8.5  1.20  6.00  •      3.60  30.60 

16 9.2  13.40  11.20  12.30  113-16 

17 10.5  5.00  11.20  8.10  85.05 

18 lo.o  7.20  17.60  12.40  124.00 

19 6.7  17.00  19.40  18.20  121.94 

23 II.4        4.00        5.00        4.50       51.30 

21 4,6        3.60        4.40        4.00       18.40 

22 lO.O       15.40        9.20       12.30      123.00 

^2i 5.0  21.60  30.20  25.90  129.50 

24 10.7  20.00  17.00  18.50  197-95 

25 8.4  16.00  12.80  14.40  120.96 

26 8.0  31.20  20.80  26.00  208.00 

27 9.0  13.20  20.40  16.80  151.20 

28 6.4  6.80  7.60  7.20  46.08 

29 6.2  13.60  16.00  14.80  91.76 

30 8.0  6.00  6.40  6.20  49.60 

31 9.9  20.00  8.60  14.30  141.57 

Totals    236.7      $1,486.40      $1,562.20      $1,524.30    $10,068.23 

Averages    ...       7.6  47.95  50.39  49.17 

In  the  first  place,  the  assays  were  made  on  two  parts  of 
the  original  sample,  and  not  on  the  two  halves  of  the  final 
pulp;  therefore  the  high  results  given  by  Nos.  5  and  12 
indicate  that  there  was  this  amount  of  gold  present  in  the 
sample  as  a  whole  and  not  in  a  small  portion  of  the  pulp 
alone.  The  diflFerence  in  the  "original"  and  ''duplicate'' 
are  due  not  to  faulty  assaying,  but  to  the  fact  that  the 
two  halves  of  the  sample  actually  differed  in  their  con- 
tents. It  is  obvious  that  the  division  of  the  original 
sample  into  two,  in  this  way,  serves  the  purpose  of  test- 
ing the  distribution  of  the  values,  and  it  is  also  a  check  on 
the  high  assays  by  indicating  whether  they  are  due  merely 

—208— 


SAMPLING    AND    KSTIMAIION    OF    ORE    IN    A    MINE 

to  an  accidental  particle  of  gold  in  the  assay  pulp  or  really 
to  a  rich  spot  in  the  lode.  If  done  by  different  assayers, 
this  method  is  a  check  on  the  assaying  itself.  The  arith- 
metical mean  of  the  assays  of  the  two  halves  differs,  as 
will  be  seen,  4.8  per  cent  only ;  which,  having  regard  to 
the  character  of  the  ore,  a  free  gold  quartz,  is  most  satis- 
factory. 

On  the  figures,  as  given,  the  ore  averages  7.6  ft.  in 
width  and  $42.53  (10068.23  divided  by  236.7)  in  value  per 
ton,  which  is  about  $6.50  less  than  the  arithmetical  mean 
of  the  assays.  If,  now,  the  two  high  assays  are  omitted, 
the  average  becomes  $19.27  (that  is,  10068.23  minus 
5763.40,  divided  by  236.7  minus  13.3),  which,  having  re- 
gard to  the  fact  that  rich  spots  are  known  to  contribute  an 
integral  part  of  the  output  of  the  mine,  is  probably  too 
small.  If  the  engineer  has  the  opportunity,  he  will  re- 
sample  the  rich  spots  and  also  take  intermediate  samples 
between  Nos.  4  and  5,  5  and  6,  11  and  12,  12  and  13 — 
thereby  finding  out  for  certain  whether  the  rich  ore  does 
occur  to  the  extent  of  the  10  ft.  in  length,  which  in  this 
case  is  the  interval  between  the  samples.  In  default  of 
such  re-sampling,  as,  for  instance,  if  he  has  left  the  mine 
by  the  time  he  gets  his  assay  returns,  then  he  should  sub- 
stitute for  5  and  12  the  average  obtained  by  omitting  the 
two  high  assays  and  also  the  very  poor  samples,  such  as 
Nos.  2,  8,  14,  15,  16,  17,  18,  20  21  22,  28  and  30.  This 
will  give  an  average  of  $27.70  for  the  richer  ore  as  ob- 
tained by  subtracting  5763.40  (3275.40  and  2488)  from 
10068.23,  and  also  deducting  834.21  (the  sum  of  the  foot- 
dollars  representing  the  12  lowest  assays),  and  then  di- 
viding by  126  (236.7  less  111.7,  the  aggregate  width  of 
the  14  samples  omitted). 

Now,  substituting  $27.70  in  place  of  the  two  high  as- 
says and  putting  in  the  corresponding  foot-dollars  for 
Nos,  5  and  12  in  the  last  column,  we  get  a  final  average  of 
$19.75  per  ton,  that  is,  4673.24  ( 10068.23  minus  5763.40, 
but  plus  368.41),  divided  by  236.7.  This  $19.75  com- 
pares with  $49.17  without  any  correction  and  $19.27 
when  the  two  high  assays  only  are  omitted. 

— 209 — 


SAMPLING    AND    ESTIMATION    OF    ORE    IN,  A    MINE 

In  most  cases  the  result  obtained  in  this  way  will  be 
corroborated  by  actual  mining  and  milling.  In  this  case 
the  difference  made  by  taking  the  average  of  the  better 
grade  ore,  instead  of  taking  the  average  of  all  the  other 
samples  save  the  two  "high  assays,"  is  comparatively 
slight,  but  the  example  quoted  is  intended  chiefly  as  illus- 
trating a  method  rather  than  a  result. 

Mention  has  been  made,  by  Mr.  Bancroft,  for  instance,^* 
of  the  air  of  importance  given  to  the  supposed  greater 
accuracy  of  mill-runs  as  compared  to  the  assay  of  numer- 
ous small  samples.  This  is  a  heritage  from  an  older 
period.  It  is  obvious  that  the  bulk  of  a  sample  weighing 
several  tons  requires  that  it  shall  be  handled  by  several 
men,  and  that  this  affords  plenty  of  opportunity  for  tam- 
pering with  the  ore.  Either  the  choice  of  the  ore  broken 
can  be  made  so  as  to  favor  the  desired  result,  or  actual 
"salting"  can  be  perpetrated  with  comparative  ease. 
Furthermore,  the  number  of  ounces  of  amalgam  scraped 
off  the  plates  in  a  mill  depends  largely  on  the  previous 
conditions  of  the  plates,  that  is,  the  amount  of  gold  amal- , 
gam  which  they  carried  when  the  test  began.  If  rich  ore 
follows  poor  ore  in  a  mill,  the  plates  get  part  of  it,  and 
the  yield  is  too  low,  while  if  the  reverse  happens  and  poor 
stuff  is  milled  after  rich  ore  has  passed  over  the  plates, 
the  result  will  be  unduly  high.  In  one  case,  which  came 
under  my  notice,  several  lots  of  ore  were  broken  in  a  cer- 
tain mine  and  were  sent  to  a  custom  chlorination  works 
for  treatment,  with  results  which  ranged  between  $12  and 
$17  per  ton,  but  when  the  mine  had  been  bought,  on  the 
evidence  of  these  returns,  it  was  found  that  the  ore  yield- 
ed about  $8  per  ton.  Of  course,  in  breaking  the  sup- 
posed "samples,"  no  average  was  really  secured  because 
there  was  a  general  desire  among  all  concerned,  quite 
honestly,  to  get  a  satisfactory  yield.  As  to  the  use  of 
stamp-mills  for  such  tests,  it  is  obvious  that  there  is  great 
danger  of  both  willful  and  innocent  deception.  From 
feeding  amalgam  into  the  mortar  boxes  to  scraping  the 
plates  down  to  the  bare  copper,  there  is  every  modifica- 

**On  page   123. 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

tion.  It  is  the  old  question  of  one  big  sample  versus 
many  small  ones.  However  big  the  sample  may  be,  you 
get  one  figure  and  one  factor ;  with  many  results  from  a 
large  number  of  samples  you  average  the  data,  minimiz- 
ing the  errors  and,  often,  detect  them  by  the  mere  process 
of  comparison. 

The  discussion  has  brought  out  many  sides  of  the  prob- 
lem of  mine  valuation;  what  may  be  called  the  purely 
mechanical  side,  the  sampling  itself,  obtained  the  first 
emphasis  because  it  is  fundamental  to  all  the  mental  pro- 
cesses which  result  in  the  estimation  of  ore  in  reserve. 
Mr.  F.H.  Minard  points  out^"'  that  to  delegate  sampling  to 
inexperienced  youths  is  a  dangerous  procedure,  because 
the  carrying  out  of  the  work  involves  something  more 
than  ability  and  energy ;  the  measuring  of  ore  and  break- 
ing of  samples  requires  experience,  and  if  the  preceding 
treatment  of  the  subject  has  not  made  this  abundantly 
clear,  it  has  failed  egregiously.  Therefore  engineers 
should  not  delegate  their  work  indefinitely,  nor,  if  they 
are  wise,  will  the  younger  men  court  disaster  by  under- 
taking tasks  for  which  they  are  unfitted.  There  is  such  a 
thing  as  learning  one's  profession ;  a  man  must  be  an 
apprentice  before  he  becomes  a  master.  And  when  all 
the  sampling  and  investigation  are  done  the  great  prob- 
lem of  valuation  is  there  still,  to  try  the  mettle  of  the  en- 
gineer. It  takes  a  strong  man  to  be  a  good  valuer,  says 
my  friend  Mr.  Curie.  It  does,  indeed — the  kind  of  moral 
courage  which  faces  facts  and  sends  sophistry  to  the 
devil.  It  is  the  courage  which  animates  science  and  re- 
fuses to  make  two  and  two  anything  but  four,  neither 
making  it  three  by  over-caution  nor  swelling  it  to  five  by 
an  unhealthy  optimism.  The  engineer  whose  estimate  is 
25  per  cent  below  the  fact  is,  in  my  opinion,  as  blame- 
worthy as  the  one  who  makes  it  25  per  cent  too  high.  It 
is  well  enough  to  say  that  you  safeguard  your  client,  if  a 
buyer,  by  putting  a  low  valuation  on  a  mine,  but  there  is 
no  honesty  in  such  a  line  of  reasoning,  for  you  are  helping 
another  man  to  get  something  for  less  than  it  is  worth 


"On  page  176. 


SAMPLING    AND    ESTIMATION    OF    ORE    IJT    A    MINE 

and  giving  the  lie  to  your  professional  judgment.  To 
overvalue  a  property,  when  caught  by  the  enthusiasm  of 
tlie  seller,  is  foolish  enough,  and,  as  the  world  goes,  it 
brings  a  speedy  retribution  by  becoming  destructive  to 
reputation.  But  the  two  departures  from  fact  are  of  a 
piece;  there  is  nothing  to  choose  between  them;  and  to 
say  that  they  are  both  equally  unprofessional  is  as  much  a 
euphemism  as  it  would  be  to  say  that  it  is  unseemly  to 
tell  an  untruth. 

Every  mine  is  a  new  and  distinct  problem ;  the  valua- 
tion of  it  requires  not  only  the  determination  of  actual 
ore  measurable,  but  the  appraisal  of  future  possibilities. 
There  was  a  time  when  the  gambling  element  was  in- 
volved in  the  sampling,  for  it  was  rendered  as  haphazard 
as  the  throw  of  the  dice ;  but  since  the  thorough  testing 
of  mines  has  gjown  to  become  a  recognized  procedure, 
the  judgment  of  prospective  value  looms  up  with  added 
significance.  Mines  will  not  be  bought  and  sold,  in  the 
future,  like  a  cheese  that  has  been  tested;  they  will  be 
taken  over  for  the  speculative  enhancement  to  be  secured 
by  skillful  development.  As  Mr.  J.  Parke  Channing 
shows,^®  you  can  buy  pyrites  mines  on  the  basis  of  a  for- 
mula, in  which  certain  determined  factors  are  properly 
included ;  but  when  it  comes  to  appraising  the  prospective 
enhancement  of  gold,  copper  and  lead  mines,  for  instance, 
formulas  are  thrown  overboard,  and  there  is  need  for  that 
instinct,  judgment,  nous — call  it  what  you  will — which 
enables  one  miner  to  arrive  at  a  safe  guess,  while  the 
other  sees  no  further  than  the  head  of  his  pick.  It  is  the 
training  of  the  "tributer"  or  "leaser"  which  is  needed. 

While  the  moil  is  the  accepted  tool  for  sampling,  the 
gad  is  advocated  by  Mr.  Lee  and  the  small  pick,  but  only 
for  special  work,  by  Mr.  Walter  H.  Weed.-'  In  certain 
places  underground  a  gad,  fitted  with  a  handle,  is 
more  serviceable  than  a  moil,  that  is,  when  the 
ore  to  be  sampled  is  so  situated  that  one  cannot  hold 
a^  moil    in   the    left   hand   so   as   to   strike  it   with   the 


"See  pages  88  to  90. 
"See  pages  128  and  145, 

— 212 — 


SAMPLING   AND    ESTIMATION    OF    ORE    IN    A    MINE 

hammer  held  in  the  right.  The  gad  with  a  handle  is 
one  of  the  oldest  implements  of  the  miner,  and  figure^  in 
the  insignia  of  the  Schools  of  Mines  at  Leoben  and  Claus- 
thal.  As  to  the  small  pick,  the  geologist's  or  expert's 
pick,  as  it  would  be  termed  in  the  West,  it  is  useful  in  pre- 
liminary sampling  and  in  testing  various  parts  of  a  vein 
to  find  out  which  streaks  carry  the  values,  but  I  am  loath 
to  recommend  it  for  systematic  sampling,  especially  in 
hard  ground.  It  can  be  used  by  an  experienced  man  with 
discretion  to  good  purpose,  but  I  would  advise  the 
younger  engineers  to  leave  it  at  home  or  in  the  black- 
smith's shop.  Much  the  same  is  true  of  the  "grab"  sam- 
ple which  I  consigned  to  perdition  as  being  at  the  root  of 
so  many  untrustworthy  estimates  of  ore.  Mr.  W.  M. 
Courtis  and  Mr.  Albion  S.  Howe  say,^^  very  properly,  that 
the  grab  sample  is  serviceable  in  finding  out  which  band 
of  ore,  in  a  mt^ltiple  vein,  carries  the  values,  and  in  this 
way,  by  taking  one  or  two  grab  samples,  Mr.  Courtis  has 
saved  himself,  on  more  than  one  occasion,  from  being 
salted.  Here,  too,  for  a  specific  purpose,  this  method  is 
well  enough,  but  as  a  general  way  of  arriving  at  the  ton- 
nage and  value  of  large  bodies  of  ore,  it  is  hopelessly  un- 
scientific ;  in  fact,  the  very  notion  of  a  "grab,"  that  is,  to 
seize  blindly  at  a  handful  of  broken  ore,  with  the  idea  that 
by  being  blind  you  are  impartial,  is  wholly  opposed  to  the 
earnest  effort  to  eliminate  the  element  of  chance  as  much 
as  possible,  by  collecting  the  maximum  number  of  data, 
and  applying  to  them  the  doctrine  of  averages — which  is 
the  basis  of  systematic  sampling. 

In  regard  to  the  matter  of  rolling  sticky  ore  on  canvas 
or  duck,  as  discussed  by  Messrs.  Richard  A.Parker,  W.L. 
Austin  and  G.  M.  Gouyard,^^  any  error  due  to  this  cause 
may  arise  from  the  fact  that  rich  ore  becomes  powdered 
in  the  course  of  reduction,  especially  in  the  presence  of 
pieces  of  'gouge'  or  clay,  such  as  frequently  mark  the 
walls  of  lodes.  A  sample  of  poor  ore  may  become  'salted' 


"On  pages  129  and  130. 
i^See  pages  150,   151,   152. 


—213— 


SAMPLING    AND    ESTIMATION    t)F    ORE  "^N    A    MINE 

if  rolled  on  the  canvas  cloth  which  has  been  used  in 
quartering  a  richer  material.  The  discussion  of  this  point 
serves  to  accentuate  the  dangers  surrounding  the  quarter- 
ing of  samples ;  but  while  the  use  of  canvas  for  this  pur- 
pose is  open  to  objections,  it  still  remains  the  quickest 
way  to  accomplish  the  desired  end — of  reducing  the  bulk 
of  the  sample — in  the  absence  of  suitable  split-shovels  and 
other  apparatus,  which  are  not  readily  portable  to  distant 
mining  localities.  Wherever  convenient,  and  especially  in 
the  case  of  the  examination  of  important  mines,  I  would 
advise  the  employment  of  a  split-shovel  and  sampler,  of 
the  kind  used  in  assay  offices.-" 

In  regard  to  the  use  of  a  strip  of  canvas  to  catch  the 
ore  as  it  is  broken  instead  of  using  a  candlebox,  I  cannot 
agree  with  Mr.  Walter  Harvey  Weed,-^  because,  even  if 
intentional  'salting'  be  prevented  by  excluding  ever}'one 
save  those  engaged  in  taking  the  samples,  unintentional 
interference  with  correct  results  will  arise  from  the  falling 
of  bits  of  ore  outside  of  the  range  of  the  section  sampled. 
In  many  lodes  carrying  friable  minerals  or  pulverulent 
ore,  or  even  sugar  quartz,  which  has  been  loosened  during 
the  course  of  mining,  there  will  be  a  tendency  for  particles 
to  drop  onto  the  canvas  from  spots  not  being  sampled, 
owing  to  the  vibration  set  up  by  the  blows  of  the  hammer 
on  the  moil.  Rock  at  a  distance  will  be  jarred  loose  and 
will  get  mixed  up  with  the  sample  being  taken.  Of 
course,  if  the  candle-box  is  not  held  near  the  face  of  ore, 
the  chips  broken  off  by  the  sampler  will  fly  wide  and 
some  of  the  material  broken  will  escape  the  box,  but  this, 
with  a  little  experience,  can  be  obviated.  The  man  hold- 
ing the  box  can  put  his  hand  up  to  the  face  of  ore  so  as 
to  catch  the  broken  rock  and  the  candle-box  can  be  held 
so  as  to  arrest  the  flying  chips.  In  any  event,  to  my 
mind  the  use  of  a  canvas  is  open  to  the  objection  men- 
tioned, and  should  not  be  resorted  to  except  in  one  con- 
tingency— that  is,  when  an  engineer  is  alone,  has  no 
assistant  with  him,  and  has  reason  to  distrust  any  help 


"See  also  page  23. 

»See  page   145  and  also  page  179. 


-214- 


SAMPLING   AND    ESTIMATION    OF    ORE    IN    A    MINE 

which  is  offered  by  those  on  the  mine.  Under  such  cir- 
cumstances he  cannot  very  well  take  a  large  sample, 
without  placing  a  canvas  on  the  floor  of  the  level  or  stope 
so  as  to  catch  the  ore  broken  down  by  him,  because  it  is 
physically  impossible  for  him  to  hold  a  box  and  break  a 
sample  at  the  same  time. 

The  remarks  of  Mr.  George  A.  Packard^^  concerning 
low  assays  are  much  to  the  point.  In  ores  containing 
metals  in  quantities  large  as  compared  to  the  minute  pro- 
portion of  gold  usually  present  in  lodes,  there  is  a  tend- 
ency for  erratic  low  results  to  disturb  the  real  average 
just  as  much  as  the  "high  assay"  in  gold  mines.  But,  as 
pointed  out  already,  the  high  result  always  differs  from 
the  low,  especially  in  the  case  of  the  precious  metals,  be- 
cause the  former  may  be  due  to  an  accidental  inclusion  of 
a  small  particle  of  specimen  ore  or  a  "metallic'  in  the  final 
pulp,  while  the  latter  simply  bespeaks  an  absence  of  rich 
ore  in  the  whole  of  the  sample,  provided  the  latter  has 
been  reduced  so  that  the  assay  pulp  represents  the  average 
contents  of  the  crude  ore  broken  in  the  process  of 
sampling. 

In  regard  to  talking  straight  to  one's  client,  telling  him 
whether  the  facts  are  sufficient  to  be  the  basis  of  an 
opinion  or  not,  explaining  the  element  of  risk  and  taking 
care  to  distinguish  between  probabilities  and  possibilities 
— in  all  this  I  agree  heartily  with  the  views  expressed  by 
Mr.  Packard,-^  and,  as  the  reader  is  aware,  I  have  en- 
deavored to  emphasize  the  advisability  of  being  outspoken, 
for  business  reasons,  as  well  as  from  finer  considera- 
tions. 

The  mathematical  demonstrations  and  methods  of  cal- 
culation described  by  Mr.  Blamey  Stevens-*  and  by  Mr. 
Auguste  Mathez-^  are  interesting,  not  only  because  they 
will  facilitate  the  arrival  at  conclusions,  but  chiefly  from 
the  fact  that  they  illustrate  the  methematical  basis  for  the 
methods  which  men  reach  more  laboriously  through  ex- 

220n  page   i8i. 
^Sce  page   183. 


»*On  pages  155  to  161. 
*On  pages  161  to   163. 


—215— 


SAMPLING    AND    ESTIMATION    OF    ORE    IN    A    MINE 

perience.  Empiricism,  rule  of  thumb,  practical  ways — 
these  are  all  reached  by  a  process  of  elimination,  and,  in 
so  far  as  they  are  logical  and  survive  the  test  of  usage, 
they  can  be  expressed  in  terms  of  mathematics. 

Mr.  Gemmell  offers  several  useful  hints.-®  In  deprecat- 
ing "unnecessary  refinement"  of  method  he  exhibits  a 
natural  impatience,  for  once  a  man  is  thoroughly  experi- 
enced he  can  dispense  with  certain  details  of  procedure, 
as  a  golf  player  who  is  in  good  form  may  not  need  to 
waggle  his  club  before  making  a  drive.  But  in  describing 
all  the  possible  precautions  against  error,  I  did  so  for  the 
benefit  of  the  novice,  who,  as  he  gains  experience,  will 
be  warranted  in  dispensing  with  some  of  them,  while 
retaining  others,  according  to  the  circumstances  of  each 
case. 

The  examples  quoted  by  Mr.  Gemmell  are  full  of  sug- 
gestion; it  is  by  contact  with  such  facts  that  a  man  ac- 
quires the  judgment  which  enables  him  to  detect  the 
weak  points  of  any  estimate,  and  so  avoid  them.  No  con- 
tribution to  this  discussion  will  be  more  valuable  than 
these  transcripts  from  experience  as  given  by  Mr.  Gem- 
mell, if  they  emphasize  the  enormous  variety  of  conditions 
to  be  faced  in  the  valuation  of  mines. 

Impatience  with  an  apparently  needless  consideration  of 
niceties  of  method  has  become  apparent  in  the  latter  part 
of  this  discussion  on  sampling,  and  a  letter  received  from 
a  veteran  engineer  voices  this  sentiment  by  suggesting 
that  too  much  emphasis  is  being  placed  upon  useless  de- 
tails, which  have  the  effect  of  obscuring  the  essential 
facts. 

Well,  there  is  something  to  be  said  for  that;  an  en- 
gineer should  not  get  so  tied  up  with  his  paraphernalia 
and  methods  as  to  lose  sight  of  the  purpose  of  it  all,  for, 
if  he  doc3  that,  he  will  resemble  the  centipede  who  got 
along  very  well  on  his  multitudinous  legs  until  the  scorpion 
asked  him  which  leg  he  put  out  first,  and  thereupon  the 
centipede  became  so  self-conscious  that  his  extremities 
became  all  tangled  up  and  he  fell  by  the  wayside.    There 


^^See  page  184. 

—216— 


SAMPLING    AND    ESTIMATION    OF    ORE    IX    A    MINE 

is  a  happy  medium  between  discarding  a  proper  pro- 
cedure and  becoming  the  victim  of  it,  to  be  so  'practical' 
as  to  neglect  precautions  and  to  be  so  'technical'  as  to 
lack  sense  of  proportion.  This  entire  consideration  of  a 
difficult  part  of  mining  practice  illustrates  the  importance 
of  the  personal  factor.  The  mathematical  demonstrations 
and  occasional  short-cuts,  such  as  have  been  described 
by  Messrs.  Channing,  Hoffman,  Stevens  and  Mathez,  are 
all  suggestive  of  the  fact  that  the  methods  evolved  by  ex- 
perience, if  good,  are  logical;  but  beyond  this  there  is  an 
insistent  element  present  in  all  estimates  of  ore  in  a  mine, 
that  is,  the  personal  equation,  which  canhot  be  expressed 
by  any  algebraic  sign  save  x.  No  branch  of  professional 
work,  not  even  the  management  of  men,  requires  more 
resourcefulness  and  skill ;  for  this  reason  it  is  considered 
one  of  the  severest  tests  of  an  engineer's  capacity.  If, 
therefore,  the  importance  of  this  subject  has  become 
recognized  increasingly  during  the  last  few  years,  it  is 
good  evidence  of  the  emergence  of  mining  from  the 
mists  of  speculation  on  to  the  firm  ground  of  serious  busi- 
ness. 


— ««»— 


INDEX. 


PAGE. 

Argall,  Philip ^(i,  93,  114,  116,  119,  137,  182 

Contribution  to  Discussion 102,  1 17 

On  Ore  Developed 76 

Assay   Plans    172 

Austin,  W.  L 178,  213 

Contribution  to  Discussion 150 

Bancroft,  George  J 200,  210 

On  Unsystematic  Sampling  118 

Banks,  John  H 88 

Brown,  R.  Oilman,  Contribution  to  Discussion 14S 

On  Cost  Per  Ton  as  a  Basis  of  Mine  Valuation 82 

Brunton,   F.    W 75 

Burnham,  F 202 

Butte  Copper  Mines  "Ji 

California    Gold    Mines 150 

Camp  Bird  Mine   142,  175 

Cananea  Mines  120 

Carter,  T.  Lane  202 

On  Mine  Valuation   131 

Channing,  J.  Parke 212,  217 

Contribution   to   Discussion 192 

On  Mine  Valuation   88 

Charleton,  A.  G 75,  148 

Check  Samples  105 

Collateral  Evidence  in  Mine  Valuation 71 

Collins,  Arthur  L 93 

Collins,  George  E 96,  102,  112,  117,  200 

Contribution  to  Discussion 113 

On   Ore   Developed    93 

Cost  of  Mine  Examination 14,  170 

Cost  Per  Ton  as  a  Basis  of  Mine  Valuation 82 

Courtis,    W.    M 213 

Contribution  to  Discussion 130 

Cripple  Creek  Mines 47 

Curie,   J.   H 205 

On  Some  Aspects  of  Mine  Valuation 138 

—319— 


SA.NrriJXC,    AND    ESTIMATION    OF    ORE    IN    A    MINE 

PAGE. 

Denny,  G.  A 75,   183,  202 

Contribution   to   Discussion   163 

Erratic  Samples  1 11 

Explosives,   Use   of 18,  145 

Future  Prospects  of  a  Mine 60 

Gemmell,  R.  C 216 

Contribution  to  Discussion 184 

Gouyard,  G.  M 213 

Contribution  to  Discussion 115,  152 

Grab  Samples 34,  130 

High  Assays  39,  181,  207 

Treatment   of    40 

HoflFmann,  Ross 217 

Mathematical  Basis  of  Sampling 35 

Howe,   Albion   S 213 

Contribution  to  Discussion 130 

Jones    Sampler    108,  109 

Kendall,  J.  D. 75 

Kirby,    E.    B 75 

Lawrence,  Benjamin  B".,  Contribution  to  Discussion 116 

Lee,  Chester  F 201,  212 

Contribution    to   Discussion 128 

Levy,  Ernest   200 

On  Sampling  the  Floor  of  a  Wet  Level 124 

Low   Assays,    Influence   of 181,  215 

Mathez,  Augusta  217 

Contribution  to  Discussion   161 

McDermott,  W 75 

Method  of  Least  Squares  as  Applied  to  Sampling 157 

Meyer  &  Charlton  Gold  Mining  Co.,  Ltd.,  Sampling  Record.  168 

Minard,  F.  H 211 

Contribution  to  Discussion I75 

Mine  Valuation  131,  205 

Mining  Costs,  Determination  of  11 

Numbering  Samples 178 

Ore,  Definition  of 10 

Determination  of  Value  14,  I47>  I49 

Ore  Blocked.  Definition 115 

Ore  Developed,  Definition 93 

Ore  Expectant,  Definition 80 

Ore    Faces,    Definition 115 

Ore-Reserves,   Estimation   of 5^,  206 

Organization  of  a  Sampling  Gang 15 

Packard,   George   A 215 

Contribution   to  Discussion 181 

Panning      I35 

— 220 — 


SAMPLING    AND    ESTIMATION    OF    ORK    IX    A    MINE 

PAGE. 

Parker,  Richard  A 151,  152,  178,  213 

Contribution  to  Discussion  150,  154 

Peele,  Robert 88 

Prismoidal  Formula  as  Applied  to  Ore-Bodies 162 

Probable  Error  in  Sampling 157 

Probable  Ore,   Definition  of 117 

Positive  Ore,  Definition  of 117 

Purington,  C.   W 200 

Contribution  to  Discussion  96 

Pyrites  Mines,  Valuation  of 89 

Quartering   Samples 25,  146 

Rand  Mines 51,  61,   131,  164,  202 

Raub   Mine    141 

Reduction  of  Samples 23 

Rickard,  Forbes,  Contribution  to  Discussion 195 

Rickard,  T.  A 103,  137,  175 

Review  of  Discussion 199 

Sampling  and  Estimation  of  Ore  in  a  Mine 9 

Sample  Card  185 

Samples,  Reduction   23,  107 

Marking  Position 103 

Sacking  107 

Screening 24 

Size    206 

Sampling,  Bottom  of  Wet  Level 200 

Cost  of  170 

Inferences  from   55 

Influenced  by  Pitch  of  Lode 33 

Interval 16,  172,  201,  203 

Narrow  Veins    i77 

On  the  Rand  131,  163,  196 

Precautions    in 26 

Quarterings 195 

Record  168,  169 

Rings 186,  187 

Tools    18,  212 

Work  of   17.  105 

Wrong  Methods 31 

Sampling  and  Mining,  Discrepancies  Between 44,  136 

San  Juan  (Colo.)  Mines 62 

Screening  Samples    24 

Seven-Thirty  Mine 54 

Silver  Plume  Mine 54 

Sketching    Samples 172 

Smuggler-Union  Mine  42 


sampling;  and  estimation  of  Ore  in  a  mine 

PAGE. 

Stevens,  Blarney  217 

Contribution  to  Discussion   155 

Tin  Gravels,  Sampling  of 58 

Tomboy  Mine  42,  142 

Treadwell,  John  C 200 

Contribution  to   Discussion 1 15 

Truscott,  S.  J 75 

Unsystematic  Sampling  200 

Valuation  of  a  Prospect 183 

Of  Mines 131,  205 

Of  Mines  in  New  Districts  167 

Weed,  Walter  Harvey 179.  212,  214 

Contribution  to  Discussion   145 

Wet  Level,   Sampling  in 124,  194 

Winslow,  Arthur  1 14 

Wybergh,  W 75 


